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The    Embryology 

Anatomy  and  Histology 
of  the  Eye 


BY  EARL  J.  BROWN,  M.  D. 

Professor  of  Histology  of  the  Eye,  at  the  Chicago  Eye,  Ear,  Nose  and  Throat  College 


WITH    ILT.ITSTRATIOXS    MADE  FROM    TRANSVERSE    SECTIONS    OF   THE 
HITMAN    EVE   EXT.ARGED    BY    MICRO  PHOTOGRAPHY 


The  Physiology  of  Vision 


BY  WM.  D.  ZOETHOUT,   Ph.  D.,  (U.  of  C.) 

Professor  of  Physiology  in  thejenner  Medical  College  and  nt  the  Bennet  College 
of  Eclectic  Medicine,  Chicago.  / 


ILLUSTRATIONS   KY   THE   AU'IHOR 


CHICAGO: 

Hazlitt  Sc  Walker,  Publishers. 

1906 


Copyrighted 

11)05 

BY  HAZLITT  &  WALKER 


FOREWORD. 


In  approaching  this  work,  perhaps  a  word  of  explana- 
tion to  the  reader  may  be  desirable.  It  is  not  undertaken 
because  the  author  thinks  there  is  a  lack  of  knowledge 
about  the  eye;  neither  have  there  been  any  new  facts  dis- 
covered which  would  merit  the  production  of  these  arti- 
cles. It  is  therefore  not  the  intention  to  bring  out  any  new 
facts,  but  to  put  the  known  and  widely  scattered  facts  in  a 
more  comprehensible  form  and  to  illustrate  the  subject 
so  thoroughly  and  completely  that  it  will  be  made  more 
easy  for  the  beginner  and  more  interesting  to  those  who 
find  it  necessary  to  review  the  subject. 

All  the  illustrations  of  the  structures  of  the  eyeball  and 
smaller  structures  will  be  microphotographs  taken  from  mi- 
croscopic slides  in  the  author's  possession,  while  the  coarser 
structures  of  the  orbit  will  be  illustrated  by  drawings,  as 
these  structures  are  too  large  for  the  tissues  to  be  mounted 
on  microscopic  slides. 

The  microscopic  slides  used  to  photograph  the  foetal 
eye  are  from  the  pig  and  were  procured  at  the  Armour  pack- 
ing house  by  collecting  the  foetal  pigs  at  the  gutting  table. 
These  foetesis  ran  from  two  millimeters  to  forty  millimeters 
in  length,  and  the  mounting  of  the  slides  was  done  by  Dr. 
Slonaker,  at  the  Chicago  University. 

The  slides  used  in  photographing  the  adult  eye  were  made 
by  Dr.  Slonaker  when  he  wrote  his  thesis  on  the  acute  area 
of  vision.  These  slides  have  been  used  in  my  illustrated 
lectures  before  optical  and  medical  societies  for  several 
years,  and  they  have  been  enjoyed  so  much  by  my  hearers 
and  I  have  received  so  many  requests  for  them  in  a  i)er- 
manent  form,  that  it  is  in  response  to  these  wishes  that  the 


■iS^22 


b  FOREWORD. 

author  lias  determined  to  perpetuate  these  pictures  and 
place  them  in  the  reach  of  every  one  who  is  interested  in 
the  eye ;  otherwise  these  articles  would  never  have  appeared. 

A  few  words  about  the  physical  development  of  the  foetus 
might  be  of  benefit  before  the  illustrations  are  studied.  The 
foetus  is  first  represented  by  one  cell,  the  ovum.  This  is 
fertilized  by  the  spermatozoa;  then  there  is  a  multiplication 
of  cells.  These  increase  very  rapidly,  and  the  first  definite 
form  assumed  is  a  tube,  representing  the  worm,  and  this 
tube  has  two  walls ;  one  is  the  outer  covering  and  the  other 
lines  the  inside.  The  outer  is  known  as  the  cpiblast  (mean- 
ing above)  and  the  inner  the  hypoblast  (meaning  below). 
Then  there  is  a  layer  developed  between  these  two  layers. 
This  layer  is  known  as  the  mesablast  (meaning  the  middle). 
From  the  epiblast  is  developed  the  skin  and  nervous  sys- 
tem. From  the  hypoblast  is  developed  the  alimentary  canal 
and  all  the  internal  organs  which  communicate  with  the 
alimentary  canal.  From  the  middle  layer,  or  mesoblast, 
is  developed  the  connective  tissue,  blood  vessels,  muscles, 
bones,  etc. 

From  the  foregoing  we  see  that  in  the  study  of  the  eye 
we  are  most  especially  concerned  in  the  epiblast,  as  it 
forms  the  nervous  system  and  therefore  the  brain,  and  the 
inner  seat  or  sensory  coat  of  the  eye,  and  some  one  has 
well  said  that  the  eye  is  a  part  of  the  brain  placed  near  the 
surface,  back  of  an  opening,  where  it  may  receive  impres- 
sions from  the  external  world  and  communicate  these  im- 
pressions to  the  main  portion  of  the  brain. 

The  first  indication  of  the  nervous  system  commences 
by  the  development  of  two  ridges  along  the  dorsum,  or  back, 
of  the  foetus  during  its  tubular  development.  These  are 
known  as  the  neural  ridge$.  The  cells  composing  these 
ridges  multiply  and  they  rise  higher  and  higher  and 
finally  meet  al)Ovc,  at  the  center,  and  coalesce,  or  grow 
together,  leaving  an  opening.     Tliis  is  knowm  as  the  neural 


FOREWORD.  7 

tube,  and  the  whole  nervous  system  is  developed  from  the 
cells  which  line  this  tube. 

Soon  after  the  neural  tube  is  formed,  the  anterior  half 
of  the  foetus  folds  on  itself,  and  this  portion  forms  the 
brain,  while  the  posterior,  or  unfolded  portion,  forms  the 
spinal  cord.  From  the  anterior  portion  of  the  brain,  two 
tubes  grow  out  and  toward  the  surface.  These  are  known 
as  the  optic  stalks,  and  they  form  the  first  step  in  the  de- 
velopment of  the  eye. 


CHAPTER  I. 

•  EMBRYOLOGY. 

It  might  be  well  to  explain  that  the  major  part  of  the  em- 
bryology of  the  eye  has  been  worked  out  from  the  eye  of 
the  chick  and  rabbit,  as  it  is  almost  impossible  to  get  fresh 
material  in  human  embryos.  The  writer  conceived  the  idea 
of  going  to  a  large  packing  house,  where  hundreds  of  preg- 


Fig.  1.     Horizontal  Section  through  Head  of  Foetal  Pig,  2  mm.  long. 
Magnified  3,000  times. 


nant  sows  were  gutted  every  day  and  material  could  be  ob- 
tained fresh  and  in  all  the  stages  of  development.  This  was 
suggested  to  Dr.  J.  Rollin  Slonaker  of  Chicago  University, 
and  he,  acting  on  the  suggestion,  procured  the  material  and 
prepared  the  microscopic  slides  from  which  the  following 
illustrations  were  made. 

8 


THE    ANATOMY    OF    THE    EYE.  9 

The  first  manifestation  of  the  development  of  the  eye  is 
a  hollow  protrusion  from  that  part  of  the  neural  tube  which 
forms  the  anterior  cerebral  vesicle.  A  vesicle  is  an  enclosed 
cavity,  between  two  layers  of  tissue  and  filled  with  fluid, 
like  a  water  blister  on  the  hand.     The  neural  tube,  as  ex- 


Fig.  2.    Horizontal  Section  through  Head  of  Unhatchhjd  Chick,  2  ihtn. 
long.     Magnified  3,000  times. 


plained  before,  is  a  tube  developed  along  the  dorsum  or  back 
of  the  foetus,  during  the  tubular  stage  of  development,  and 
the  whole  nervous  system  is  developed  from  the  cells  lining 
this  tube.  This  hollow  protrusion  is  known  as  the  primary 
optic  stalk.     (See  A,  Fig.  i.) 

As  this  stalk  grows  outward,  the  anterior  portion  rises 
upward,  as  shown  in  vertical  section  at  A,  Fig.  3.  When 
the  optic  stalk  comes  near  to  the  surface,  the  anterior  por- 
tion enlarges,  as  shown  at  C,  Fig.  i.  Also  when  the  optic 
stalk  encroaches  on  the  surface,  it  stimulates  the  epithelial 
cells  forming-  the  skin  and  they  multiply  rapidly  (see  B, 
Figs.  I  and  3),  and  the  anterior  wall  of  the  primary  optic 


lO 


THE    ANATOMY    OK    THE    EYE. 


Fig.  3.     Vertical  Section  through  Head  of  Foetal  Pig,  2  mm.  long.    Mag- 
nified 2,500  times. 


tr'rjj^ 

W\d 

i^F^Bi 

zy^% 

M  1 

Ra     ''' 

\m 

^^^IkJl^H 

0^ 

ll^^^H^^f . 

Fig.  4.    Horizontal  Section  through  Head  of  Foetal  Pig,  3  mm.   long. 
Magnified  about  1,200  times. 


THE    ANATOMY    OF    THE    EYE. 


II 


vesicle  invaginates  and  passes  inside  of  the  vesicle.  (See 
A,  Fig.  2,  and  C,  Fig.  3.)  This  invagination  might  be 
likened  to  the  denting  of  a  hollow  rubber  ball. 

This  invaginated  portion  forms  the  secondary  optic  vesicle 
and  it  is  from  this  that  the  nine  innermost  layers  of  the 
retina  are  eventually  formed,  while  the  primary  optic  vesicle 
only  forms  the  outer  or  pigment  layer.     As  the  secondary 


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£«.• 

Fiy.  5.    Vertical  Section  thr()u.i^li  Head  of.Foetal  Pi^,  3  mm.  long.    Mag- 
nified about  2,000  times. 


Optic  vesicle  is  passing  into  the  primary  optic  vesicle  there 
appears  in  front  of  the  secondary  optic  vesicle  a  depression 
on  the  surface  at  the  point  of  activity  of  the  epithelial  cells. 
(See  A,  Fig.  4,  and  A,  Fig.  5.)  This  depression  becomes 
deeper  and  deeper  and  the  mouth  is  finally  closed  by  the 
rapid  formation  of  cells  around  the  depression,  as  shown 
at  B  and  C,  Figs.  4  and  5. 


THE    ANATOMY    OF    THE    EYE. 


Fiii.  6. 


Vertical  Section  through  Head  of  Foetal  Pig,  4  mm.  long.    Mag- 
nified about  1,000  times-: 


Fig.  7.    Horizontal   Section  through   Head  of  Foetal  Pig,  7  mm.   long. 
Magnified  about  600  times. 


THE    ANATOMY    OF    THE    EYE. 


13 


Til  US  a  vesicle  is  formed  which  is  known  as  the  lens 
vesicle,  as  shown  at  A,  Fig.  6.  Then  this  vesicle  hecomes 
separated  from  the  surface  and  passes  into  the  secondary 
optic  vesicle  (see  A,  Fig.  7),  and  eventually  forms  the  lens, 
which  will  be  described  later. 


Fig.  8.    Vertical  section  through  head  of  pig,  8  mm.  long.     Magnified 

460  times." 

As  the  lens  vesicle  passes  into  the  secondary  optic  vesicle, 
some  of  the  mesoblastic  cells  pass  upward  from  below,  be- 
hind it  (see  B,  Fig.  6),  and  it  is  these  mesoblastic  cells  which 
will  eventually  multiply  and  form  the  vitreous  body.  It  will 
be  remembered  that  the  mesoblast  is  the  middle  layer  of  the 
three  primary  layers,  first  formed  in  the  foetus.  The  sur- 
face of  the  skin  from  which  the  lens  vesicle  was  cut  away, 
remains  and  forms  the  cornea  and  some  students  of  the 
embryology  of  the  eye  believe  that  the  cornea  owes   its 


H 


THE    ANATOMY    OF    THE    EYE. 


transparency  to  the  changes  that  take  place  in  the  nature  of 
the  cpitheHal  cells  during  the  formation  of  the  lens  vesicle 
from  this  immediate  point.  The  lids  are  formed  by  an  ex- 
ternal fold,  growing  downward  from  above  and  upward 
from  below  the  eyeball  and  the  first  indication  of  this  growth 
is  shown  at  B,  Fig.  7. 


Fig.  9.    Horizontal  section  through  head  of  pig,  9 

460  times. 


long.    Magnified 


The  further  development  of  these  folds  is  shown  at  A 
Figs.  8  and  9,  also,  there  is  a  groove  running  from  the 
inner  side  of  the  eye  to  the  nasal  cleft  of  the  foetus  and  the 
edges  of  this  groove  come  together  and  cover  in  the  cells 
at  the  bottom  of  this  groove  and  a  cord  is  formed  from  the 
nasal  cleft  to  the  palpebral  fissure.  (The  palpebral  fissure 
is  the  opening  between  the  lids.)  This  cord  divides  and 
one  branch  goes  to  the  upper  lid  and  the  other  to  the  lower. 
Later  there  is  a  tube  formed  from  this  cord  and  this  tube 
so  formed  is  the  lachrymal  or  tear  duct,  which  runs  from 
the  palpebral  fissure  to  the  nose,  and  it  is  through  this  duct 


THE    ANATOMY    OF    THE    EYE.  I5 

that  the  tears  are  pumped  from  the  conjunctival  sack  to  the 
nose.     This  process  will  be  explained  later. 

In  Figs.  7  and  8  the  lens  vesicle  will  be  seen  to  have  been 
entirely  separated  from  the  surface,  and  at  B,  Fig.  8,  is 
seen  the  epithelial  cells  which  will  form  the  outer  layer  of 
the  cornea,  and  at  C,  Fig.  8,  is  seen  the  mesoblastic  cells 
which  will  multiply  and  eventually  form  the  four  innermost 
layers  of  the  cornea,  the  iris  and  other  structures  anterior 
to  the  lens. 

At  D,  Fig.  8,  is  seen  the  commencement  of  the  formation 
of  the  lens  substance.  This  formation  is  accomplished  by 
the  cells  of  the  posterior  portion  of  the  lens  vesicle  wall 
elongating  and  forming  long  spindle  cells.  These  grow 
forward  and  fill  the  whole  cavity  of  the  lens  vesicle,  as 
shown  at  D,  Fig.  9,  and  these  extend  from  the  anterior  to 
the  posterior  limits  of  the  cavity  and  are  known  as  the  lens 
fibers.  At  E,  Fig.  8,  is  seen  the  opening  at  the  posterior 
pole  of  the  eye  ball,  where  the  axis  cylinder  processes  make 
their  escape  from  the  eye  ball,  as  shown  at  E,  Fig.  9,  to 
pass  into  the  optic  nerve  as  they  grow  from  the  retina  to- 
ward the  brain.  This  opening  through  which  the  optic 
fibers  leave  the  eye  ball  is  known  as  the  choroidal  fissure 
in  the  adult  eye. 

At  F,  Figs.  8  and  9,  are  shown  the  mesoblastic  cells  which 
have  passed  into  the  space  between  the  retina  and  the  lens. 
As  shown  at  B,  Fig.  6,  they  are  just  commencing  to  form 
the  vitreous  body,  and  this  cavity  so  filled  is  known  as  the 
vitreous  cavity  in  the  adult  eye.  At  G,  Figs.  8  and  9,  the 
primary  optic  vesicle  is  shown,  which  has  become  quite  thin, 
and  in  the  cells  forming  it  there  is  being  deposited  pigment 
granules,  and  it  will  be  remembered  that  this  eventually 
forms  the  outer  or  pigment  layer  of  the  retina.  At  H, 
Figs.  8  and  9,  will  be  seen  the  first  indication  of  the  forma- 
tion of  the  nine  innermost  layers  of  the  retina,  and  these 
nine  layers  are  all  formed  from  the  walls  of  the  secondary 
optic  vesicle.  There  is  a  folding  over  of  the  optic  stalk 
and  optic  vesicles,  which  is  well  illustrated  by  the  accom- 
panying diagrammatic  drawing,  Fig.  10. 


l6  THE    ANATOMY    OF     THE    EYE. 

A  represents  the  primary  optic  vesicle;  B,  tlic  secondary 
optic  vesicle;  C,  the  walls  of  the  primary  optic  stalk,  and 
D,  the  groove  below  the  optic  stalk.     The  lower  edges  of 


n 


Fig.  10. 

the  optic  stalk  come  together  at  E.  and  coalesce,  thus  form- 
ing a  tube  with  a  double  wall  which  extends  from  the  eye 
ball  to  the  cranial  cavity.  This  joining  together  takes 
place  clear  forward,  along  the  lower  part  of  the  primary 
and  secondary  optic  vesicles  to  F.  Thus  an  eyeball  is 
formed  and  the  fissure  closed  is  known  as  the  choroidal 
fissure  of  the  foetus.  However,  at  the  posterior  of  the  eye- 
ball there  is  an  opening  left,  through  which  the  axis  cylinder 
processes  leave  the  eyeball,  E,  Figs.  8  and  9.  This  opening 
is  known  as  the  choroidal  fissure  in  the  adult  and  corre- 
sponds to  the  optic  disc  as  seen  with  the  ophthalmoscope. 
It  is  this  folding  over  of  the  embryonic  structures  of  the  eye 
which  makes  it  possible  for  the  incorporation  of  the  arteria 
centralis  retina  (central  artery  of  the  retina)  and  its  accom- 
panying vein  within  the  optic  nerve  for  some  distance  back 
of  the  eye,  in  the  adult,  as  this  artery  was  already  developed 
in  the  groove  below  the  optic  stalk.  H,  Fig.  10,  represents 
the  lens  vesicle  within  the  secondary  optic  vesicle.  Fig.  11 
represents  a  vertical  cross  section  of  the  primary  and  sec- 
ondary optic  vesicles  at  about  the  line  marked  G,  in  Fig. 
10,  and  A  in  Fig.  11  shows  the  primary  optic  vesicle  wall. 


THE    ANATOMY    OF    THE    EYE.  1 7 

B,  Fig.  II,  shows  the  secondary  optic  vesicle  wall  and  C 
shows  the  choroidal  fissure  at  the  bottom  of  the  foetal  eye. 
D,  Fig.  II,  is  the  vitreous  cavity.  F,  Fig.  ii,  is  the  lens 
vesicle  cut  through,  and  the  two  edges  which  come  together 
and  close  the  choroidal  fissure  are  shown  at  E.  When  this 
union  fails  to  take  place  we   have  an  anomaly  known  as 


Fig.  11. 

colobom^a  of  the  fundus  in  the  adult  and  means  a  lack  of 
development. 

At  A,  Fig.  12,  will  be  seen  the  further  development  of  the 
lids;  B,  Fig.  12,  the  anterior  epithehal  layer  of  the  cornea, 
and  just  beneath  it  is  seen  a  lighter  colored  Hne.  This  is 
the  anterior  homogeneous  (structureless)  layer  of  the 
cornea,  also  known  as  Bowman's  membrane,  as  he  was  the 
first  to  describe  it.  C,  Fig.  12,  shows  the  lamina  propria 
(proper  layer)  of  the  cornea.  D,  Figs.  12  and  13,  shows 
the  lens  fibers  extending  from  the  front  to  the  back  of  the 
lens.  These  fibers  are  simply  long  spindle  cells  and  each 
one  has  a  nucleus.  These  form  a  crescent-shaped  line  of 
dots,  as  seen  at  K,  Figs.  12  and  13,  running  from  one  side 
of  the  lens  to  the  other.  At  J,  Figs.  12  and  13,  is  seen 
the  transitional  (transformation)  zone,  and  it  is  at  this 
point  that  the  lens  fibers  are  formed,  and  this  formation  is 
simply  the  multiplication  of  the  columnar  epithelial  cells, 
which  first  formed  the  wall  of  the  lens  vescicle  and  their 


i8 


TTTK    ANATOMY    OF    TITE    EYE. 


Fig.  12.    Horizontal  sectioti  through  ej'e  of  a  pig.    Magnified  730 tinres. 


Fig.  13.    Human  embryo  eye,  2  months.     Magnified  1,080  times. 


THE    ANATOMY    OF    THE    EYE. 


19 


eluiii4atioii  into  spindle  cells.  These  spindle  cells  are  known 
as  the  lens  fibers.  These  fibers  are  especially  well  illus- 
trated at  D,  Fig.  15,  and  anterior  to  this  transitional  zone 
where  the  lens  fibers  are  formed  in  the  adult  eye  will  be 
found  a  single  layer  of  the  columnar  epithelial  cells.  Un- 
derneath the  capsule  L,  Figs.   12  and  13,  and  J,  Fig.   16, 


Fig.  14.    Eye  of  embryo  pig,  10  mm.  long.      Magnified  1,600  times. 


while  posterior  to  the  transitional  zone,  no  such  cells  will 
be  found,  they  having  elongated  to  form  the  lens  fibers, 
as  shown  at  D,  Figs.  8  to  15. 

After  the  lens  vesicle  is  completely  filled  by  fibers,  ex- 
tending from  the  front  to  the  back  of  the  lens  in  an  anterior- 
posterior  direction,  as  shown  by  the  lens  in  Figs.  9  to  13, 
there  is  a  continuation  of  growth  of  the  lens  by  the  multi- 
plication and  elongation  of  the  columnar  cells  at  the  transi- 
tional zone,  J,  Figs.   12,  13  and  14.     These  grow  forward 


20 


THE    ANATOMY    OF    THE    EYE. 


and  backward  toward  the  anterior  and  posterior  poles  of 
the  lens,  around  the  ends  of  the  first  formed  fibers,  and 
these  latterly  developed  fibers  form  the  soft  outer  or  cortical 
portion  of  the  lens,  B,  Fig.  15.  While  the  first  formed 
fibers  constitute  the  nuclear  or  central   denser  portion  of 


Fig.  15.    Highly  uiaguitied  deaiii  from  the  anterior  of  a  liumaii  euibrj'o, 


the  lens,  C,  Fig.  14,  these  last  formed  fibers  grow  in  such 
a  way  that  when  their  ends  come  into  apposition  at  the 
front  and  back  of  the  lens,  there  are  formed  seams,  as  shown 
at  A,  Figs.  14  and  15.  A,  Fig.  14,  is  at  the  posterior 
surface  of  an  embryo  pig,  and  A,  Fig.  15,  is  a  more  highly 
magnified  seam  from  the  anterior  of  a  human  embryo. 
These  seams  have  a  star  or  stilate  shape,  the  central  part 
being  at  the  anterior  and  posterior  poles  and  the  points  run 
outward  toward  the  equator  of  the  lens,  and  it  is  these 
seams,  where  the  ends  of  the  fibers  in  the  cortical  portion 
of  the  lens  abut  against  each  other,  that  forms  the  so-called 


THE    ANATOMY    OF    THE    EYE.  21 

lens  stars.  These  fibers  of  the  lens  are  long,  diamond- 
shaped,  spindle  cells,  and  these  are  arranged  in  lamella  or 
layers  and  all  held  together  by  a  matrix  of  jellatinous 
cement  substance,  and  when  a  lens  is  macerated  (soaked) 
in  an  alkaline  solution,  which  will  dissolve  this  cement  sub- 
stance, these  lamella  of  the  lens  may  be  peeled  ofif,  and  the 
best  illustration  is  the  peeling  of  the  layers  of  an  onion. 
At  E,  Fig.  12,  is  seen  the  commencement  of  the  growth 
of  the  third  eye  lid,  known  as  the  membrana  nictatans 
(winking  membrane)  in  the  lower  animals,  especially  birds. 
This  develops  in  man  up  to  a  certain  stage,  then  ceases 
and  remains  as  a  vestige  in  a  crescent-shaped  fold  near  the 
inner  side  of  the  eye,  and  is  called  the  plica  semilunaris 
(half  moon  fold).  At  F,  Fig.  12,  is  seen  the  developing 
vitreous  body  and  the  dark  spots  are  the  small  blood  vessels 
which  furnish  this  body  its  nutrition  during  development. 
These  are  from  the  hyaloid  artery,  which  will  be  described 
later,  and  these  atrophy  before  birth.  At  G,  Fig.  12,  it 
will  be  seen  that  the  brim  or  fornix  at  the  anterior  margin 
of  the  primary  and  secondary  optic  vesicles  are  in  appo- 
sition to  the  capsule  of  the  lens  at  its  equator,  and  this 
enables  some  of  the  connective  tissue,  which  binds  the  retina 
together,  which  is  known  as  the  fibers  of  Mueller  (he  being 
the  first  to  discover  them)  to  become  attached  to  the  capsule 
of  the  lens,  and  as  the  eye  enlarges  and  the  retina  settles 
farther  backward,  these  attached  fibers  elongate  and  thus 
the  suspensory  ligament  (also  known  as  the  zonule  of  Zinn) 
is  formed,  and  this  accounts  for  this  connection  between  the 
retina  and  the  lens. 

At  H,  Fig.  12,  will  be  seen  the  farther  development  of 
the  layers  of  the  retina.  At  I,  Fig.  12,  are  seen  some  cells, 
which  are  showing  signs  of  activity.  This  is  the  first  sign 
of  the  development  of  the  choroid  and  sclerotic  coats. 

At  A,  Fig.  16,  it  will  be  seen  that  the  lids  are  gradually 
covering  the  cornea  and  the  membrana  nictatans.  E,  Fig. 
16,  is  not  any  farther  developed  than  seen  in  Fig.  12  at  E. 


22 


THE    ANATOMY    OF    THE    EYE. 


At  B,  Fig.  1 6,  will  be  seen  a  portion  of  the  hyaloid  artery. 
This  is  an  artery  given  off  by  the  arteria  centralis  retina 
at  the  head  of  the  optic  nerve  and  only  exists  during  foetal 
life  lor  it  atrophies  before  birth.  It  supplies  the 
nutrition   necessary   for   the   development   of   the   vitreous 


Fig.  16.   Horiaontal  section  through  head  of  pig,  20  inni.  long.  Magnified 

670  times. 


body  and  the  lens  and  when  these  are  fully  ma- 
tured it  atrophies  and  the  canal  through  which  it  passed 
remains  as  a  lymph  channel  and  is  known  as  the  hyaloid 
canal,  or  the  canal  of  Stilling,  in  the  adult  eye.  The  hyaloid 
artery,  as  before  stated,  is  a  branch  of  the  arteria  centralis 
retina  and  runs  from  the  head  of  the  optic  nerve  to  the  pos- 
terior of  the  lens,  giving  off  small  twigs  to  the  developing 
vitreous   body.      At   the   posterior   surface   of   the   lens   it 


tUE    ANATOMY    OF    THE    EYfe. 


-Fiij.  IT.     Horizontal  section  through  head  of  a  pig,  25-  mm.  long.    Mag- 
nified 85  times. 


H 


THE    ANATOMY    OF    THE    EYE. 


breaks  up  into  several  branches.  These  pass  around  the  lens 
to  the  front  and  there  come  together,  forming  anastomoses 
(an  anastomosis  is  where  one  vessel  runs  into  another  and 
continues  by  continuity  of  tissue),  and  the  connective  tissue 


Fig.  18.  Horizontal  section  through  head  of  pig,  40  mm.  long.  Magnified 

225  times. 


which  these  vessels  are  imbedded  in  forms  the  pupilary 
membrane,  which  will  be  described  later.  There  is  a  con- 
nection of  these  hyaloid  arteries  by  anastomosing  vessels 
from  the  front  of  the  iris,  near  its  free  margin,  with  the 
branches  of  the  blood  vessels  of  the  iris.  This  connection 
only  exists  during  foetal  life. 


THE    ANATOMY    OF    THE    EYE. 


25 


At  F,  Fig.  16,  is  shown  a  band  of  tissue  connecting  the 
lens  with  the  retina.  This  will  eventually  form  the  suspen- 
sory ligament  or  the  Zonule  of  Zinn  of  the  older  writers. 
At  H,  Fig.  16,  is  shown  the  farther  development  of  the  cells 


Fig.  19.    Vertical  section  through  head  of  pig,  40  mm.  long.    Magnified 

320  times. 


which  will  eventually  form  the  choroid  and  sclerotic  and  it 
will  be  noted  that  they  may  be  traced  well  into  the  optic 
nerve  (C,  Fig.  i6),  at  either  side  at  the  choroidal  fissure, 

and  it  is  these  cells  which  will  form  the  lamina  cribrosa 


26 


THE    i\NATOMY    OF    THE    EYE. 


(seive  layer),  which  strengthens  the  eye  ball  at  this  point 
in  the  adult  eye.  Also  C,  Fig.  i6,  shows  the  first  growth  of 
the  axis  cylinder  processes  through  the  choroidal  fissure 
to  form  the  optic  nerve.     It  must  be  remembered  that  the 


Fijj.  20.    Horizontal  section  through  eye  of  a  pig,  50 
nified  150  times. 


iin.  long.    Mag- 


fibers  which  transmit  impulses  of  the  sight  from  the  retina 
to  the  brain,  grow  from  the  cells  in  the  ganglionic  layer  of 
the  retina,  back  toward  the  brain  and  not  from  the  brain  to 
the  retina.  At  I,  Fig.  i6,  is  shown  the  activity  of  the  cells, 
which  are  just  commencing  to  form  the  recti  (straight)  or 
extrinsic  muscles  of  the  eye.    At  K,  Fig.  i6,  will  be  seen  a 


THE    ANATOMV    OF 


[IE    EYE. 


27 


line  of  small  openings.  This  is  the  commencement  of  the 
space  of  Tenon.  Fig.  17  is  a  horizontal  section  through 
the  head  of  a  pig,  25  M.  M.  long,  and  is  shown  to  illustrate 
the  rapid  development  of  the  eyes  in  the  growth  from  9  to 
25  M.  M.  in  length.  A,  Fig.  17,  shows  the  nasal  cavities. 
C,  Fig.  17,  shows  the  developing  brain,  and  D  shows  the 


Fig.  21.      Vertical  section   through  human  foetal  eye  at  five   months 
Magnified  120  times. 


developing  bone.  At  E  is  seen  the  farther  development  of 
the  extrinsic  muscles,  at  F  is  shown  the  farther  develop- 
ment of  the  choroid  and  sclerotic  and  at  G  is  seen  the 
plica  semilunaris  and  at  H  the  lids. 

At  A,  Figs.  i8  and  19,  are  shown  the  lids  entirely  cover- 
ing the  front  of  the  eye  ball  and  just  back  of  it  the  cornea  B, 
and  the  space  between  the  two,  C,  is  the  conjunctival  sack. 


28  THE    ANATOMY    OF    THE    EYE. 

The  conjunctiva  lines  the  inner  surface  of  the  lids,  then 
folds  on  itself  as  shown  at  D.  This  is  known  as  the  Fornix 
(arch)  conjunctiva.  Then  it  covers  all  the  front  exposed 
portion  of  the  eye  ball  except  the  cornea.  The  epithelial  layer 
of  the  conjunctiva,  continues  over  the  cornea  and  forms  the 
outermost  or  stratified  epithelial  layer  of  this  structure,  E, 
Figs.  i8  and  19.  That  portion  of  the  conjunctiva  lining  the 
lids  is  known  as  the  palpebral  conjunctiva,  F.  Figs.  18  and 

19,  and  that  portion  covering  the  exposed  scleral  portion  of 
the  eye  ball  is  known  as  the  ocular  conjunctiva,  as  shown  at 
G.  At  H,  Fig.  18,  is  shown  the  plica  semilunaris,  and  it 
will  be  noted  that  it  is  gradually  becoming  smaller.  At  I, 
Figs.  18  and  19,  is  shown  the  choroid,  which  is  just  form- 
ing; at  J,  is  shown  the  sclerotic  and  at  K,  Figs.  18  and  19, 
is  shown  the  farther  development  of  the  extrinsic  muscles. 

When  the  twt*  lids  come  into  apposition  in  front  of  the 
eye  ball  they  become  cemented  together,  as  shown  at  h. 
Fig.  19.  In  all  animals  in  which  the  retina  is  completely 
developed  before  birth  the  lids  are  separated  at  birth,  but 
in  those  animals  whose  retina  is  not  fully  developed  at  birth, 
such  as  the  kitten  and  puppy,  the  lids  do  not  separate  for 
some  days  after  birth,  or  until  the  retina  is  sufficiently  de- 
veloped so  as  to  withstand  the  effects  of  light. 

At  A,  Fig.  20,  is  shown  the  conjunctival  sack,  at  B  the 
shrinking  plica  semilunaris  and  at  C  the  tendon  of  the  ex- 
ternal rectus  muscle  and  its  attachment  to  the  eye  ball  in 
front  and  the  belly  of  the  muscle  posteriorly.     At  D,  Fig. 

20,  is  shown  the  sheath  of  the  optic  nerve  and  the  farther 
development  of  the  nerve  itself,  at  E  the  vitreous  body  and 
at  F  it  will  be  seen  that  the  retina  is  farther  developed  and 
about  four  layers  may  be  made  out. 

At  A,  Fig.  21,  is  shown  the  developing  fibers  of  the 
orbicularis  (circular)  palpebrarum  muscle,  at  B  is  shown 
the  margins  of  the  lids  and  the  developing  cilia  (hairs)  or 
eye  lashes,  and  at  C  is  shown  a  developing  hair  in  the  lid. 
D,  Fig.  21,  shows  the  commencement  jf  the  development 


THE    ANATOMY    OF    THE    EYE. 


29 


of  the  ciliary  body  and  the  iris.  These  are  the  last  structures 
to  be  developed  within  the  eye  ball.  E,  Fig.  21,  shows  the 
cut  end  of  the  inferior  oblique  muscle,   F  shows  the  lens 


^^^^^^  ^^^^r^>  iBBB|^^^»___H 

^^^^^^^^^^^^^^^Hja      ^BkiB#°'^^ 

IT' 

^         ~]^    ^^V  m 

^T/'W^ 

Fig.  22.      Vertical  section  through  eye  of  pig,  110  mm.  long.    Magnified 

480  times. 


fully  developed,  also  the  fibers.  G,  Fig.  21,  shows  the 
vitreous  body  and  H  shows  the  retina  practically  as  is  found 
in  the  adult  eve. 


30  THE    ANATOMY    OF    THE    EYE. 

A,  Fig.  22,  shows  the  pupilary  membrane  stretching 
across  the  pupilary  space,  and  in  it  may  be  seen  Httle  white 
areas.  These  are  the  branches  of  the  hyaloid  artery  which 
furnishes  the  nutrition  to  the  lens  during  its  development, 
and  it  will  be  remembered  that  this  artery  atrophies  before 
birth  and  that  the  pupilary  membrane  disappears,  ostensi- 
bly being  absorbed.  At  B,  Fig.  22,  is  shown  the  iris  grow- 
ing out  from  the  ciliary  bodies.  C  and  D  shows  the  cornea 
and  in  it  is  shown  the  lacuna  (small  lakes),  which  are 
minute  openings  between  the  layers  of  the  lamina  propria 
(proper  layer),  and  E  shows  the  lid  with  its  developing 
structures.  F,  Fig.  22,  shows  the  conjunctival  sack  and  G 
shows  the  ocular  conjunctiva  and  just  back  of  it  the  anterior 
portion  of  Tenon's  space.  H,  Fig.  22,  shows  the  levator 
palpebra  superioris  (the  lifter  of  the  upper  lid).  I  shows 
the  lids  held  together  by  the  cement  substance  and  J  shows 
the  vitreous  body  (glass-like  body). 


CHAPTER  II.. 

ANATOMY. 

Having  hitrriedl}'  described  the  development  of  the  eye 
ball,  we  will  Jiow  go  over  the  adult  eye,  giving  the  gross 
and  leaving  the  minute  anatomy  until  we  have  advanced 
farther  with  the  subject.  The  adult  eye  ball  is  24.5  mm. 
across,  24.  mm.  from  front  to  back,  23.5  from  top  to  bottom, 
weighs  a  fraction  less  than  one-quarter  ounce  and  is  com- 
posed of  the  segments  of  two  spheres ;  the  anterior  portion, 
or  the  cornea,  A,  Fig.  23  (meaning  hor^ilike),  being  the 
segment  of  a  much  smaller  sphere  than  the  posterior  or 
scleral  portion,  the  cornea  comprising  one-sixth  of  the  outer 
surface,  while  the  sclerotic  (hard  or  tough),  shown  at  B, 
makes  up  the  other  five-sixths.  The  cornea  is  transparent 
and  thus  forms  the  window  through  which  the  light  is  ad- 
mitted to  the  eye  ball  and  this  transparency  allows  us  to 
see  the  iris  (rainbow),  E,  the  structure  lying  directly  behind 
the  cornea.  The  iris  is  a  circular  structure  pierced  at  the 
center  by  the  opening  known  as  the  pupil.  It  contains  two 
muscles,  the  one  surrounding  the  pupil,  which  is  a  narrow 
band  of  circular  fibers  known  as  the  sphincter  pupillae  mus- 
cle (meaning  the  bijider  muscle),  K.  This  muscle  closes 
the  pupil,  to  protect  the  delicate  tissues  at  the  back  of  the 
eyeball  from  bright  or  intense  light,  then  the  dilator  pupillae 
muscle,  the  fibers  of  which  extend  from  the  base  of  the  iris 
to  the  sphincter  pupillae.  This  muscle  enlarges  the  pupil 
when  more  light  is  required  to  form  a  denser  picture  on  the 
retina.  The  lens,  F,  lies  just  back  of  the  pupil  but  can  only 
be  seen  after  it  has  lost  its  transparency.  Continuing  back- 
ward from  the  base  of  the  iris,  will  be  seen  the  ciliary  body, 

31 


32  THE    ANATOMY    OF    THE    EYE. 

I,  and  between  this  structure  and  the  sclerotic  is  found  the 
ciliary  muscle,  H.  In  front  of  the  ciliary  muscle  and  at  the 
base  of  the  iris,  is  seen  the  pectinate  ligament  (comblike 
ligament),  Q  and  J.  This  is  made  up  of  many  small  bun- 
dles of  connective  tissue,  running  from  the  periphery  of  the 
cornea  to  the  base  of  the  iris,  across  the  angle  formed  by  the 
junction  of  the  cornea  and  the  iris.  This  angle  is  known  as 
the  filtration  angle,  for  the  aqueous  fluid,  which  fills  the  an- 
terior and  posterior  chambers,  leaves  the  eyeball,  at  this 
point.  It  passes  into  the  spaces  of  fontana  (fountain 
spaces),  the  spaces  of  fontana  simply  being  the  space  be- 
tween the  bundles  of  fibers  forming  the  pectinate  ligament, 
and  from  these  spaces  the  aqueous  fluid,  or  nutrient  lymph, 
as  it  is  sometimes  called,  passes  through  the  tissues  to  the 
canal  of  Schlemm,  which  is  seen  in  Fig.  23  in  the  cornea 
just  outside  of  the  spaces  of  fontana.  The  canal  of 
Schlemm  is  a  circular  channel  within  the  corneal 
tissue,  extending  clear  around  the  periphery  of  the 
cornea  and  the  fluids  pass  from  the  canal  of 
Schlemm  to  the  anterior  ciliary  veins.  Extending  backward 
from  the  ciliary  bodies  and  continuous  with  them,  are  the 
ciliary  processes.  These  end  near  the  ora  serratta  (saw 
tooth  mouth),  X,  of  the  retina.  Running  from  the  ora  ser- 
ratta forward  to  the  lens,  imbedded  in  the  outer  layer  of 
the  hyaloid  membrane  and  bound  down  firmly  to  the  inner 
surface  of  the  ciliary  processes  and  bodies  is  the  suspensory 
ligament  or  Zonule  (belt)  of  Zinn,  as  Dr.  Zinn  first  de- 
scribed it,  G.  The  ligament  proper  is  made  up  of  very 
elastic  fibers,  which,  as  before  stated,  are  imbedded  in  the 
outer  layer  of  the  hyaloid  membrane.  The  hyaloid  mem- 
bra^ie  surrounds  the  vitreous  body  and  these  fibers,  the 
writer  believes,  to  be  elongated  fibers  of  Mueller,  which  be- 


THE    ANATOMY    OF    THE    EYE.  33 

came  attached  to  the  lens  during  foetal  life  when  the  fornix 
(arch)  of  the  primary  and  secondary  optic  vesicles  were  in 
apposition  (touching)  to  the  equator  of  the  lens  and  as  the 


Fig.  23.    Cross  section  of  the  Eye,  showing  its  construction. 


globe  enlarged  they  elongated.  See  Figs.  7  to  20.  This 
ligament  leaves  the  ciliary  bodies  and  passes  across  the 
space  between  them  and  the  le,ns,  a  part  of  the  fibers  passing 
a  little  anterior  of  the  equator  and  the  rest  a  little  posterior 
to  the  equator  of  the  lens  and  are  attached  to  the  capsule 


34  THK    ANATOMY    OF    THE    EYE. 

of  the  lens.  The  triangular  space  formed  by  this  separation 
of  the  suspensory  ligament  fibers  is  known  as  the  Canal  of 
Petit,  shown  at  R.  The  lens,  F,  is  a  transparent  body  and 
occupies  the  space  just  back  of  the  iris  and  between  the  cir- 
cle of  inward  projecting  ciliary  bodies.  It  is  round,  and 
flattened  from  before  backward,  its  anterior  and  posterior 
surfaces  being  convex,  the  posterior  surface  having  the 
shorter  radius  of  curvature.  It  lies  in  a  depression  m  the 
anterior  surface  of  the  vitreous  body.  This  depression  is 
known  as  the  fossae  Patellaris  (dishlike  depression)  and  is 
supported  by  this  and  the  suspensory  ligament.  The  lens  is 
surrounded  by  a  dense  transparent  membrane  known  as  the 
capsule.  The  space  in  front  of  the  ciliary  bodies,  suspen- 
sory ligament  and  lens,  and  back  of  the  iris,  is  known  as  the 
posterior  chamber,  T,  and  the  space  in  front  of  the  iris  and 
lens  at  the  pupilary  space  and  behind  the  cornea,  is  known 
as  the  anterior  chamber,  S. 

The  sclerotic  coat  (tough  coat),  B,  continues  backward 
from  the  cornea  by  continuity  (continuation  of  tissue  by 
blending  one  into  another)  of  tissue  over  the  posterior  five- 
sixths  of  the  eyeball  to  the  optic  nerve,  where  it  divides,  the 
inner  portion  forming  the  lamina  cribrosa  (sieve  layer),  M, 
whilst  the  outer  portion  passes  into  the  sheath  of  the  optic 
nerve  Y.  It  is  pierced  by  the  ciliary  arteries  P;  by  nerves 
which  enter  the  eyeball  in  a  circle  surrounding  the  nerve; 
by  the  vena  vortacosa,  four  or  five  of  which  leave  the 
eyeball  just  back  of  the  equator;  and  by  the  anterior  ciliary 
arteries  and  veins  which  enter  the  eyeball  at  the  attach- 
ments of  the  extrinsic  muscles,  just  back  of  the  cornea. 
The  region  where  the  cornea  ends  and  the  sclerotic  begins 
is  known  as  the  limbus  (seam),  W,  and  the  angle  or  de- 
pression formed  by  the  difference  in  the  radius  of  curva- 
ture of  the  two  spheres,  represented  in  the  formation  of  the 
eyeball  in  the  corneal  and  scleral  portion  Z,  is  known  as  the 
sclero  corneal  sulsus  (furrow).  This  angle  makes  the  eye- 
ball stronger  and  more  firm  at  this  point  and  it  is  just  inside, 


THE    ANATOMY    OF    THE    EYE.  35 

Opposite  lliis  angle  that  the  cihary  muscle,  II,  is  attached 
anteriorly,  whilst  posteriorly  the  longitudinal  fibers  are  at- 
tached to  the  outer  surface  of  the  choroid,  in  the  region  of 
the  ciliary  processes  and  bodies,  as  this  muscle  is  interposed 
between  the  sclerotic  and  choroid  in  this  region.  The  ciliar)' 
muscle,  H,  is  made  up  of  two  sets  of  muscular  fibers,  the 
longitudinal  nuining  antero-posteriorly  which  are  placed 
farthest  out,  next  to  the  sclerotic,  and  the  circular  fibers 
which  lie  farthest  inward,  just  outside  of  the  ciliary  bodies. 
These  last  named  fibers  take  a  circular  course  ajid  form  a 
band  of  circular  fibers  extending  entirely  around  the  ciliary 
ring. 

Just  inside  of  the  ciliary  muscle  and  sclerotic  is  found  a 
very  vascular  pigmented  layer,  C,  knowii  as  the  choroid 
(meaning  membrane).  This  is  loosely  attached  to  the 
sclerotic  by  the  exchange  of  bundles  of  tissue  called  tra- 
beculae  and  this  space  so  formed  is  known  as  the  supra 
choroidal  space.  The  choroid  is  the  middle  tunic,  or  coat, 
of  the  three  grand  tunics  of  the  eyeball.  It  is  extremely 
vascular  and  it  is  analogous  to  the  pia  mater  of  the  brain. 
The  choroid,  ciliary  processes,  ciliary  bodies  and  the  iris 
constitute  what  is  known  as  the  uveal  coat  (grape  skin 
coat) ,  and  the  three  combined  line  all  the  scleral  portion  and 
compose  the  iris  or  curtain  in  front  of  the  lens.  Posterior- 
ly the  choroid  is  pierced  by  the  optic  nerve  and  this  opening 
is  known  as  the  choroidal  fissure  (choroidal  opening).  As 
before  stated,  the  posterior  ciliary  arteries  ajid  nerves  pass 
through  the  sclerotic  to  reach  the  choroid.  Here  the  short, 
posterior  ciliary  arteries,  P,  from  twelve  to  twenty  in  num- 
ber, divide,  one  branch  running  toward  the  optic  nerve ;  the 
others  run  anteriorly  and  begin  to  subdivide  as  they  run 
forward  supplying  the  choroid,  and  some  branch  to  the 
sclerotic.  Two  of  the  interjial  branches  may  be  seen  near 
the  optic  nerve  in  Fig.  23,  the  final  destination  of  the  an- 
terior branches  being  the  ciliary  bodies,  where  they  form 
capillary   loops   and   turn   backward   as   venous   capillaries. 


36  THE    ANATOMY    OF    THE    EYE. 

These  capillaries  keep  joining  with  others  and  forming  con- 
stantly   larger    veins,    till    finally    there    are   great    whorls 
formed  in  the  region  of  the  equator,  where  great  numbers 
join  to  form  the  vena  vortacosa  which  leave  the  eyeball  just 
back  of  the  equator  to  empty  into  the  ophthalmic  vein.  Close 
inspection  of  this  layer  in  Fig.  23  will  reveal  minute  white 
spots  al)  through  its  expanse  and  these  white  spots  are  cross 
sections  of  the  arteries  and  their  branches  as  well  as  the 
veins  of  the  whorls   from  which   the  vena  vortacosa   are 
formed    within    the    tissue.    There    are    two    long    pos- 
terior    ciliary     arteries     which     enter     the     eyeball     with 
the  short  set  of  arteries;    one  enters  just  i.nside,  the  other 
just  outside  of  the  optic  nerve.    These  pass  forward  in  the 
choroid  without  giving  off  any  branches,  until  they  reach 
the   ciliary   region.     Here  they  each   divide   into  branches 
which   take   a   circular   course   and    form    a   circle   of   an- 
astomosis at  the  base  of  the  iris  and  form  what  is  known  as 
the  circulus  major  (the  largest  circle),  2,  of  the  iris.    The 
anterior   ciliary   arteries   also   join   in   this   network,   form- 
ing an  anastomosis  with  them ;  then  from  this  outer  or  larger 
circle  branches  pass  into  the  iris  and  run  toward  the  free 
margin  or  pupil,  and  when  these  reach  the  region  of  the 
sphincter  pupillae  muscle,  another  circle  of  anastomosis  is 
formed  and  this  is  called  the  circulus  m.lnor  (smallest  circle) 
of   the  iris ;   from   this   smaller    circle  are  given   off  capil- 
laries, which  form  a  circle  of  loops  right  at  the  free  margin 
of  the  iris.    These  turn  back  as  capillary  loops,  run  one  into 
another   and  become   larger   and   larger   and   finally   form 
veins  known  as  the  anterior  ciliary  veins  ajid  these  veins 
also     receive    the    aqueous    humor    from    the    canal    of 
Schlemm,  and  therefore  drain  the  anterior  chamber.     This 
was  proven  by  injecting  coloring  matter  into  the  anterior 
chamber,  then  after  a  few  moments  killing  the  animal  and 
finding  this   colored  matter  in   the  anterior   ciliary  veins. 
The  anterior  ciliary  veins  leave  the  eye  ball  at  the  muscular 


THE    ANATOMY    OF    THE    EYE.  37 

attachments  and  pass  away  from  the  eye  ball  in  the  mus- 
cles finally  reaching  the  ophthalmic  vein  from  them. 

The  ciliary  nerves,  about  twenty  in  number,  which  arise 
from  the  ciliary  ganglion  (knot),  enter  the  eyeball  in  a 
circle  just  outside  of  the  optic  nerve.  They  run  forv/ard 
in  the  supra  choroidal  space,  giving  off  branches.  Sup- 
playing  this  structure,  as  well  as  the  sclerotic, 
they  run  forward  and  form  the  ciliary  plexus,  which  lies 
in  the  ciliary  muscle.  From  this  plexus  branches  run  to  the 
iris  and  cornea,  supplying  motor  impulses  to  the  sphincter 
pupillae  muscle,  dilator  pupillae  muscle,  as  well  as  trophic 
and  sensory  functions  to  the  iris  proper ;  the  branches  pass- 
ing to  the  cornea  are  trophic  and  sensory  only. 

Just  outside  of  the  optic  nerve,  where  it  pierces  the  eye- 
ball, is  ^und  a  circle  of  anastomosis,  giving  a  pretty  free 
blood  supply  to  the  sheath  at  this  point  and  sending 
branches  into  the  substance  of  the  nerve,  to  supply  nutrition 
to  the  sustentacular,  or  binding  tissue,  which  forms  tra- 
beculae  (beams)  between  the  nerve  bundles.  This  circle.  O, 
is  known  as  the  circulus  of  Zinn,  as  he  was  the  first  to 
describe  it. 

Passing  to  the  inner  surface  of  the  wall  of  the  eyeball. 
we  find  the  third  of  three  grand  tunics  known  as  the  ret- 
ina (net),  D.  This  lines  the  inner  wall  from  the  head  of 
the  optic  nerve,  also  called  the  optic  disc,  or  papillae,  to  the 
era  serratta.  It  is  made  up  of  seven  layers  of  nervous  tis- 
sue, two  layers  of  connective  tissue  and  one  single 
layer  of  columnar  pigmented  cell:^.  The  nine  innermost 
layers  are  held  together  by  the  sustentacular  or  binding  tis- 
sue, which  is  known  as  the  fibers  of  Muller.  The  outer  or 
pigmented  columnar  layer  is  intimately  attached  to 
the  choroid,  while  the  other  nine  layers  are 
loosely  attached  to  this  layer,  yet  firmly  attached 
to  the  choroid  at  the  ora  serratta,  while  the  ar- 
rangement of  the  uorvc  fiber  kiycr  and  the  passing  of 
the  axis  cylinder  processes  through  the  choroidal  fissure  and 


38  THE    ANATOMY    OF    THE    EYE. 

their  continuation  into  the  optic  nerve  bind  the  retina  down 
firmly  at  this  point.  The  retina  is  the  nervous  tunic  and 
the  most  sensitive  in  the  eyeball  and  is  the  one  v^hich 
makes  possible  the  sense  of  sight.  Its  most  sensitive  area 
Hes  just  outside  of  the  optic  nerve  and  is  known  as  the 
macula  lutea,  V  (the  yellow  spot),  so  named  from  the  fact 
that  if  examined  after  death,  it  will  be  seen  to  have  a  yel- 
lowish hue.  Then  again  the  central  spot  within  the  macula 
is  known  as  fovea  centralis  (or  central  pit).  The  retina 
thins  down  ajid  leaves  a  cone-shaped  pit,  there  being  only 
two  layers  at  this  central  spot.  The  retina  receives  its 
blood  supply  from  the  arteria  centralis  retina  (central  ar- 
^^^y)y  3-  Ihis  enters  the  eyeball  in  the  substance  of  the 
optic  nerve,  having  become  incorporated  in  the  nerve  during 
the  folding  of  the  optic  stalk  and  vesicles  durijig  foetal  life. 
See  Figs.  10  and  11.  When  it  passes  through  the  choroidal 
fissure  it  divides,  one  branch  passing  upward,  the  other 
downward.  These  are  known  as  the  superior  and  inferior 
branches.  Each  subdivide,  making  four  branches ;  one  run- 
nijig  upward  and  toward  the  nose,  another  upward  and 
toward  the  temple,  another  downward  and  inward  toward 
the  nose,  and  another  outward  and  downward  toward  the 
temple  and  from  the  direction  taken  they  are  named.  The 
one  running  upward  toward  the  ^lose  is  known  as  the  supe- 
rior nasal  branch,  whilst  the  one  running  downward  to- 
ward the  nose  is  known  as  the  inferior  nasal ;  the  one  run- 
ning upward  toward  the  temple  is  known  as  the  superior 
temporal,  the  one  running  downward  toward  the  temple 
is  known  as  the  inferior  temporal  branch.  The  farther  sub- 
divisions become  so  small  and  are  so  inconstant  in  their 
arrangement,  that  they  have  never  beeji  named.  These  ves- 
sels are  imbedded  in  the  retina,  ramifying  in  the  four  inner- 
most layers.  They  are  readily  seen  with  an  ophthalmoscope 
from  the  fact  that  the  retinal  tissue  surrounding  them  is 
transparent.  These  vessels  keep  dividing  lill  (hey  become 
capillaries  and  turji  back  as  venous  capillaries.    These  capil- 


THE    ANATOMY    OF    THE    EYE.  39 

laries  keep  joining  and  rejoining  until  the  vena  centralis 
retina  is  formed  and  this  passes  out  by  the  side  of  the  arteria 
centralis  retina.  These  veins  are  normally  about  one-third 
larger  than  the  arteries  and  as  they  carry  vejious  blood, 
which  is  loaded  with  waste  products,  they  are  of  a  darker 
red  color  when  viewed  with  an  ophthalmoscope. 

As  before  stated  the  sclerotic  coat  posteriorly  divides  into 
three  parts,  the  outer  portion  continuing  into  the  sheath  of 
the  optic  nerve,  Y,  the  middle  portion  passes  to  the  pial 
sheath,  while  the  innermost  portion  breaks  up  into  bundles 
and  bridges  across  the  space  just  back  of  the  choroidal  fis- 
sure, passing  through  the  optic  nerve  and  as  these  fibers 
come  from  all  points  and  pass  across  in  all  directions,  there 
is  formed  a  sieve-like  kyer  which  is  known  as  the  lamina 
cribrosa  (sieve  layer).  This  reinforces  the  globe  at  this 
point,  which  otherwise  would  not  stand  the  strain  exerted 
by  the  normal  tension  within  the  eyeball.  The  optic  iierve 
fibers  pass  through  the  meshes  in  this  sieve  layer  and  the 
optic  nerve  proper  commences  just  back  of  this,  where  the 
insulation  in  the  form  of  the  myelin  (marrow)  sheaths  be- 
gin. The  opening  through  the  lamina  cribrosa,  through 
which  the  arteria  centralis  retina  and  veins  pass,  is  known 
as  the  porus  opticus.  At  the  head  of  the  optic  nerve,  at 
the  inner  wall  of  the  eyeball,  there  is  found  a  shallow,  fun- 
nel-shaped pit,  L,  known  as  the  physiological  cup  (nor- 
mal cup).  This  pit  is  formed  owing  to  the  fact  that  when 
the  axis  cylinder  processes  reach  the  choroidal  fissure  and 
turn  backward  over  the  edge  of  the  choroid,  they  make  a 
gradual  symmetrical  turn,  instead  of  running  out  and  mak- 
ing a  sharp  right  ajigled  turn,  so  the  innermost  fibers  join 
at  the  center,  after  having  bent  to  a  certain  extent,  thus 
leaving  this  normal  depression.  This  depression  of  course 
is  filled  by  the  vitreous  body. 

The  space  surrounded  by  the  retina,  ciliary  processes, 
ciliary  bodies,  suspensory  ligament  and  lens,  is  filled  by  the 
vitreous  body,  U.    This  is  made  up  of  shapeless  cells,  more 


40  THE    ANATOMY    OF    THK    EYE. 

to  be  compared  to  an  open  meshed  sponge  than  anything 
else,  and  fluid  and  the  whole  body  is  of  the  consistency  of 
the  white  of  an  egg.  It  is  surrounded  by  the  hyaloid  mem- 
brane, which  lies  on  the  injier  limiting  membrane  of  the 
retina.  At  the  ora  serratta,  this  hyaloid  membrane  divides. 
The  outermost  layer  is  firmly  attached  to  the  inner  surface 
of  the  ciliary  processes  and  bodies  and  passes  from  the  cil- 
iary bodies  to  the  lens,  and  imbedded  in  it  are  the  fibers  of 
the  suspensory  ligament.  The  innermost  layer  continues 
over  the  front  of  the  vitreous  body  and  lines  the  fossae 
patillaris  (dish-like  depression),  in  which  the  lens  rests. 
The  vitreous  body  and  its  surrounding  membrane  are  per- 
fectly transparent.  Running  forward  from  the  head  of  the 
optic  nerve  to  the  posterior  of  the  lens,  is  a  lymph  space, 
known  as  the  hyaloid  canal,  or  the  canal  of  Stilling;  this 
was  the  channel  through  which  the  hyaloid  artery  passed  to 
supply  nutrition  to  the  developing  vitreous  and  lens,  during 
foetal  life.  See  Fig.  i6.  This  artery  atrophies  before  birth, 
and  leaves  this  canal.  The  cornea,  aqueous  humor,  lens 
and  vitreous,  form  the  refractive  media  of  the  eye,  from 
the  fact  that  they  are  transparejit  and  are  of  different  den- 
sities and  different  curvatures,  so  arranged  that  light  enter- 
ing a  normal  eye  is  brought  to  a  focus  at  the  retina. 

The  eyeball  has  numerous  lymph  spaces  and  channels. 
The  space  between  the  sclerotic  and  choroid  is  known  as  the 
supra  choroidal  space.  The  greater  portion  of  the  contents 
of  the  eyeball  are  fluids,  which  are  practically  the  same  as 
lymph  found  in  other  parts  of  the  body;  they  are  furnished 
by  the  osmosis  (passing  out),  of  the  fluids  of  the  blood 
through  the  walls  of  the  capillaries  in  the  ciliary  bodies.  A 
portion  passes  into  the  canal  of  Petit  and  back  into  the  vitre- 
ous body,  while  the  rest  passes  into  the  posterior  chamber, 
part  directly  from  the  anterior  portion  of  the  ciliary  bodies 
and  part  from  the  canal  of  Petit.  The  supra  choroidal  space 
is  filled  with  fluids  ajid  is  drained  by  the  lymph  spaces  ac- 
companying the  vena  vortacosa.     Tn  healthy  eyes  all  these 


THE    ANATOMY    OF    THE    EYE.  4 1 

fluids  are  constantly  being  supplied  and  rapidly  passing  out, 
so  they  do  not  become  stagnant. 

The  orbits  are  four  sided  and  pyramidal  in  form.  The 
base  is  formed  by  the  brim  of  the  orbit,  A,  Fig.  24.  The 
apex  is  at  the  sphenoidal  fissure  or  opening,  shown  at  B. 
The  opening  at  the  brim  of  the  orbit,  transversely,  is  one  and 
one  half  inches,  while  vertically  it  is  but  one  and  one-fourth 
inches.  Its  depth,  from  the  brim  to  the  sphenoidal  foramen, 
is  one  and  three-fourths  inches.  The  roof  arches  somewhat 
and  the  floor  is  slightly  depressed,  while  the  outer  and  inner 
walls  are  straight.  The  walls  of  the  orbit  are  formed  by 
seven  bones.  The  roof  is  mainly  formed  by  the  orbital  plate 
of  the  frontal  bone,  shown  at  C,  and  a  very  small  portion 
at  the  posterior  of  the  orbit  by  the  lesser  wing  of  the 
sphenoid,  shown  at  D.  The  inner  wall,  from  before  back- 
ward, is  formed  by  the  nasal  process  of  the  superior  maxil- 
lary, shown  at  E,  lachrymal  F,  ethmoid  H,  orbital  process  of 
the  superior  maxillary  G  and  the  orbital  portion  of  the 
sphenoid  I.  The  floor  is  formed  by  the  orbital  plate  of  the 
superior  maxillary  J,  orbital  process  of  the  plate  K  and  a 
small  portion  of  malar  L.  The  outer  wall  is  formed  by  the 
greater  wing  of  the  sphenoid  M,  and  the  orbital  process  of 
malar  N. 

The  openings  in  the  walls  in  the  orbital  cavity  are  as  fol- 
lows :  On  the  interior  wall,  from  before  backward,  the  lachry- 
mal canal,  leading  to  the  nasal  cavity,  through  which  the 
lachrymal  duct  passes ;  the  anterior  and  posterior  ethmoidal 
foramen  (opening),  through  which  the  nasal  branch  of  the 
ophthalmic  nerve  and  artery  leave  the  orbit ;  at  the  apex  the 
sphenoidal  fissure,  through  which  the  third,  fourth,  sixth  and 
ophthalmic  branches  of  the  fifth  nerve  enter  the  orbit  and 
the  ophthalmic  vein  leaves  it;  above  and  to  the  inner  side  of 
the  sphenoidal  fissure  is  found  the  optic  foramen  O.  It  is 
through  this  opening  that  the  second  or  optic  nerve  and  the 
ophthalmic  artery  enter  the  orbit.  At  the  lower,  outer  side, 
is  found  the  spheno  maxillary  fissure  P.     It  is  through  this 


42 


THE    ANATOMY    OF    THE    EYE. 


Fig.  24.    Tlie  Human  Skull. 


Opening  that  the  upper  branch  of  the  superior  maxillary  or 
middle  division  of  the  fifth  or  trifacial  nerve  enters  the  orbit. 
It  lies  in  a  groove  in  the  floor  of  the  orbit  at  Q,  and  leaves 
the  orbit  with  the  infra  orbital  artery  through  the  infra 
orbital  foramen  R. 


THE    ANATOMY    OF    THE    EYE.  43 

Above  the  orbit,  at  its  brim,  is  found  a  small  opening, 
known  as  the  supra  orbital  foramen,  shown  at  S,  through 
which  the  supra  orbital  nerve  and  artery  leave  the  orbit. 
Sometimes  this  fails  to  fill  in  with  bone  at  the  brim  and  then 
only  forms  a  notch,  as  shown  at  T.  The  inner  walls  are 
practically  straight,  from  before  backward,  while  the  outer 
walls  run  obliquely  backward  and  inward.  Thus 
it  will  be  seen  that  the  axial  poles  of  the  two  orbits  diverge 
something  like  thirty  degrees.  The  two  eyeballs  occupy  the 
anterior  central  portion  of  the  orbits.  The  rest  of  the  orbit 
is  filled  with  the  orbital  fat  and  the  structures  necessary  for 
the  performance  of  ocular  functions  and  protection  to  the 
eyeball. 

Covering  the  front,  or  base  of  the  orbit  and  in  front  of  the 
eyeball,  are  found  the  two  lids,  the  upper  and  the  lower, 
known  as  the  palpebral  and  shown  at  G  and  H,  Fig.  25. 
The  opening  between  the  two  lids,  through  which  the  eye- 
ball is  seen  is  known  as  the  palpebral  fissure  and  where  the 
two  lids  join,  at  the  outer  and  inner  sides  of  the  eyeball,  is 
called  the  outer  and  inner  canthus,  as  shown  at  A  and  B. 
Near  the  inner  canthus,  the  two  lids  approach  one  another, 
then  separate  again  slightly,  before  coming  together,  and  this 
little  circular  portion  of  the  palpebral  fissure  is  known  as  the 
lakus  (meaning  small  lake,  and  is  so  called  because  the  tears 
flow  into  it  before  leaving  the  palpebral  fissure).  Lying 
within  the  lakus  is  a  small,  red  body,  formed  of  mucous 
tissue  and  of  some  few  very  fine  hairs,  also  the  remains 
of  the  schneiderian  gland,  which  is.  found  in  those  lower 
animals  which  have  a  third  eyelid  or  nictitating  membrane. 
This  body  is  called  the  caruncle  (small  growth  of  flesh), 
shown  at  C,  and  just  outside  of  the  caruncle  is  found  a  fold 
of  the  conjunctiva  (which  membrane  lines  the  lids  and  cov- 
ers all  the  portion  of  the  eyeball  which  is  exposed  when  the 
lids  are  parted,  except  the  corneal  portion).  This  fold  is 
the  remains  of  the  mem1)rana  nictatans  and  is  called  the  plica 
semilunaris  (half  moon  fold),  and  is  shown  at  F.    All  along 


44 


THE    ANATOMY    OF    THE    EYE. 


the  free  margin  of  the  lids,  there  is  a  row  of  hairs,  which 
extend  forward,  with  a  slight  turning  upward  at  the  outer 
ends  on  the  upper  lid  and  downward  on  the  lower  lid.  These 
are  the  cilia  (hairs)  or  eyelashes. 

As  before  stated,  when  the  lids  approach,  near  the  inner 
canthus,  they  arch  away  from  each  other,  to  form  the  lakus 


—  A 


D         a 


Fig.  25. 


and  on  the  free  margin  of  the  lids  at  this  angle,  is  found  a 
small,  slightly  raised  pdint,  known  as  the  lachrymal  papillae 
(tear  pimples),  shown  at  I,  from  the  fact  that  in  the  center 
of  each  one  is  found  a  little  opening,  called  the  lachrymal 
puncta  (minute  opening),  so  named  from  the  fact  that  the 
tears  pass  out  of  the  palpebral  fissure  through  these  two 
openings.  At  the  anterior  central  portion  of  the  eyeball  is 
seen  a  round,  dark  area,  shown  at  D,  with  a  central,  smaller, 
round  and  darker  area,  shown  at  E.  Tlie  outer,  lighter  por- 
tion, is  the  iris,  and  the  smaller,  darker  portion  is  the  opening 


THE    ANATOMY    OP^    THE    EYE.  45 

through  its  center,  known  as  the  pupil.  These  are  seen 
through  the  transparent  cornea,  M,  and  all  the  opaque,  or 
white  portion  of  the  eyeball,  seen  from  ni  front,  is  the 
sclerotic,  L,  which  is  seen  through  the  transparent  con- 
junctiva. When  the  lids  are  separated,  there  is  seen  above 
the  palpebral  fissure,  a  fold  of  skin,  J,  which  is  caused  by  a 
bundle  of  fibers  from  the  muscle  which  raises  the  upper  lid 
passing  outward  and  being  attached  to  the  skin,  which  draws 
the  lower  part  of  the  skin,  covering  the  lid,  upward  and  al- 


Fi^.  lit).    Showin/j  Tendo  Oculi. 

lowing  the  skin  covering  the  upper  part  of  the  lid  to  drop 
down,  forming  the  fold,  and  in  this  way  nature  has  provided 
against  this  loose  skin  dropping  over  the  edge  of  the  lid  and 
obscuring  vision,  when  the  Hd  is  raised  and  the  skin 
slackened. 

Above  the  orbit,  and  covering  a  ridge,  is  a  growth  of  hairs 
called  the  supra  cilia  (the  hairs  above)  or  eyebrows,  K. 
This  ridge  is  known  as  the  supra  ciliary  ridge  and  is  caused 
by  a  ridge  of  bone  and  a  muscle  underlying  the  skin.  If  the 
skin  were  dissected  away,  immediately  beneath  it  would  be 
found  the  superficial  facia  covering  the  deeper  structure  of 
the  lids  and  stretching  across  the  orbit.  This  is  a  thin, 
fibrous  sheet,  which  is  found  immediately  beneath  the  skin 


4^  THE    ANATOMY    OF    THE    EYE. 

and  areolar  tissue  in  all  portions  of  the  body.  At  the  outer 
and  inner  sides  of  the  palpebral  fissure,  running  from  the 
canthi  to  the  orbital  walls,  is  seen  the  external  and  internal 
angular  or  palpebral  ligaments,  also  called  the  orbicular  liga- 
ments (shown  at  A  and  B,  Fig.  26),  and  just  above  the  orbit 
would  be  seen  the  corrugator  supra  ciliary  muscle  (supra 
ciliary  wrinkler)  shown  at  C.  It  arises  from  the  frontal 
bone  near  the  median  line  and  along  the  supra  ciliary  ridge, 


Fig.  27.    Showing  Orbicularis  Muscle. 

and  is  attached  to  the  upper  and  outer  fibers  of  the  orbicu- 
laris muscle.  It  is  the  contraction  of  this  muscle  which 
causes  the  vertical  wrinkles  in  the  skin  at  the  lower  central 
portion  of  the  forehead.  Its  nerve  supply  comes  from  the 
facial  nerve,  yet  there  seems  to  be  a  reflex  action  between 
this  muscle  and  those  of  accommodation,  for  we  see  this 
corrugation  or  wrinkling  most  frequently  in  those  who  are 
hyperopic. 

If  we  dissect  away  the  superficial  facia,  immediately 
beneath  it  will  be  found  the  orbicularis  palpebrarum  muscle 
(circular  muscle  of  the  lids)  shown  at  D,  Fig.  27.  It  arises 
from  the  bony  walls  of  the  orbit  at  the  brim.    The  bundles 


THE    ANATOMY    OF    THE     EYE.  47 

of  fibers  pass  inward  and  take  a  circular  course  and  surround 
the  palpebral  fissure  C,  being  continuous  around  the  two 
canthi,  A  and  B.  This  muscle  is  supplied  by  the  facial 
nerve,  and  its  action  is  to  close  the  palpebral  fissure  and 
bring  the  free  margins  of  the  lids  into  apposition  (touch- 
ing), thus  hiding  the  eyeball. 

If  the  dissection  is  continued  deeper,  the  deep  facia  would 
be  exposed  and  in  the  region  of  the  eye  it  is  quite  dense  and 
fibrous  and  is  called  the  ligament  of  Lockwood.    It  is  shown 


Fig.  28.    Showing  Ligament  of  Lockwood. 

at  A,  Fig.  28.  In  it  are  embedded  the  tarsal  (lid)  carti- 
lages, and  above  will  be  found  the  levator  palpebrae  superi- 
oris  muscle  (the  lifter  of  the  upper  lid),  shown  at  B.  This 
muscle  arises  from  the  ligament  of  Zinn,  which  surrounds 
the  optic  foramen;  it  runs  forward  and  upward  and  its 
tendon  spreads  out  fan-shaped  and  is  attached  to  the  upper 
edge  of  the  tarsal  cartilage;  a  few  fibers  pass  out  and  are 
attached  to  the  skin.  Its  nerve  supply  is  from  the  third,  or 
motor  oculi. 

At  the  upper,  inner  side  of  the  orbit,  is  seen  the  trochlea 
(pulley),  shown  at  C,  and  passing  through  it  and  turning 
outward  and  downward,  to  be  attached  to  the  eyeball,  is 
seen  the  superior  oblique  muscle  D.     It  arises  also  from 


48 


THE    ANATOMY    OF    THE    EYE. 


the  ligament  of  Zinn,  passes  forward,  upward  and  inward 
through  the  orhit,  then  becomes  tendonous  and  passes 
through  the  trochlea,  then  runs  outward,  down- 
ward and  backward,  and  is  attached  to  the  eyeball  under- 
ucath  and  outside  of  the  superior  rectus  muscle,  just  back 
of  the  equator.  This  muscle  receives  its  nerve  supply  from 
the  fourth  or  patheticus  nerve.  At  the  upper,  outer  side  of 
the  orbit  is  seen  the  lachrymal  gland  (tear  gland),  shown 


B  M 

Fig.  29.    Showing  Arteries  of  the  Lids. 


at  E.  This  is  a  compound  racemose  gland  (resembling  a 
bundle  of  grapes),  and  its  ducts  empty  into  the  conjunc- 
tival sac,  at  the  fornix  conjunctiva  (arch  of  the  conjunc- 
tiva), at  the  upper,  outer  angle.  This  gland  secretes  the 
tears  which  are  poured  into  the  conjunctival  sac,  when  the 
eye  is  irritated,  to  wash  away  any  foreign  substance  which 
may  be  the  cause  of  the  irritation.  This  gland  is  especially 
supplied  with  sensory  nerves  from  the  branch  of  the  ophthal- 
mic nerve,  which  is  named  after  the  gland.     At  the  outer 


THE    ANATOMY     OF    THE    EYE.  49 

and  inner  cantlii  are  again  seen  the  angular  ligaments  F, 
and  beneath  the  internal  angular  ligament,  is  found  the 
tensor  tarsi  muscle,  which  is  supplied  by  the  facial  nerve. 
If  the  structures  of  the  lids  were  dissected  away,  leaving 
only  the  arteries,  their  arrangement  would  be  about  as  seen 
in  Fig.  29.  A  is  the  angular  artery,  the  terminal  branch  of 
the  facial,  and  it  is  through  this  branch  that  collateral  cir- 
culation  to  the  brain  is  established,  if  the  internal  carotid  is 


Fig.  30.     Showing  Veins  of  the  Lids. 

occluded,  for  it  forms  an  anastomosis  with  the  frontal  artery 
G,  this  being  the  terminal  branch  of  the  ophthalmic  arteiy. 
B  is  the  infra  orbital  artery  which  comes  to  the  surface  from 
the  orbit,  through  the  infra  orbital  foramen.  D  is  the  supra 
orbital  which  comes  from  the  orbit  to  the  face  through  the 
supra  orbital  foramen.  H  is  the  lachrymal  branch  of  the 
ophthalmic  artery,  after  piercing  the  lid.  I  shows  a  branch 
of  the  anterior  temporal  artery  as  it  comes  to  the  region  of 
the  eye.  This  branch  is  of  importance,  from  the  fact  that  in 
acute  inflammations  of  the  orbit,  or  its  contents,  leeching 


50 


THE    ANATOMY    OF    THE    EYE. 


is  resorted  to  on  the  temple,  and  it  is  the  blood  from  this 
artery  that  is  taken.  E  shows  a  branch  from  the  transverse 
facial  artery.  Running  across  the  lids,  just  above  and  below 
the  opening,  are  seen  two  arterial  trunks,  F  and  J.  They 
are  divided  into  four  arteries,  the  superior  internal  palpe- 
bral, the  superior  external  palpebral,  the  inferior  internal 
palpebral  and  the  inferior  external  palpebral.  It  will  be  seen 
that  the  lids  are  well  supplied  with  blood  and  that  there  is  a 
free  anastomosis  of  these  vessels  in  and  around  the  eyelids. 


Fig.  .U.    Showing  Nerves  of  the  Lids, 


Should  all  the  structures  of  the  lids  be  dissected  away, 
leavuig  only  the  veins.  Fig.  30  would  be  a  fair  representa- 
tion. The  names  of  these  veins  are  the  same  as  those  of 
the  arteries.  A  is  the  angular ;  B  the  infra  orbital ;  C  shows 
the  veins  draining  the  palpebral  margins,  which  are  sup- 
plied by  the  four  palpebral  arteries;  D  shows  the  frontal, 
which  forms  the  anastomosis  and  is  the  branch  through 
which  all  parts  of  the  orbit  are  drained,  if  there  is  occlu- 
sion of  the  ophthalmic  vein,  near  the  cavernous  sinus,  at  the 
back  of  the  orbit.     E  points  out  the  infra  orbital  and  F  the 


THE    ANATOMY    OF    THE    EYE.  5 1 

anterior  Iciiiporal.  Thus  it  is  seen  that  the  drainage  from 
the  lids  is  abundant  and  this  explains  why  it  is  that  inflam- 
matory conditions  in  this  region  are  so  easily  controlled 
with  hot  or  cold  compresses. 

If  all  other  structures  of  the  lids  were  dissected  away, 
leaving  the  nerves  only,  Fig.  31  would  give  a  fair  idea  of 
their  arrangement.  At  A  is  seen  the  supra  orbital  nerve, 
after  having  emerged  through  the  supra  orbital  foramen. 
At  B,  just  outside  of  it,  is  seen  the  lachrymal  nerve,  after 
having  pierced  the  lid,  and  at  C  are  seen  four  branches 
coming  from  the  facial  nerve  to  supply  the  orbicularis  pal- 
pebrarum. These  are  the  only  motor  nerves  shown  in  Fig. 
31.  The  rest  are  all  sensory  nerves  and  are  branches  from 
the  first  and  second  divisions  of  the  trifacial  or  fifth  nerve. 
At  D  is  seen  the  infra  orbital  nerve  after  emerging  from 
the  infra  orbital  foramen.  It  is  the  upper  branch  of  the 
middle  division  of  the  trifacial  nerve.  At  E  are  seen  two 
branches  emerging,  the  upper  one  passes  above  the  trochlea 
and  is  known  as  the  supra  trochlear,  while  the  lower  passes 
below  the  trochlea  and  is  called  the  infra  trochlear  nerve. 
The  aggregation  of  small  branches  near  the  free  margins  of 
the  upper  and  lower  lids  at  F,  is  known  as  the  plexus  of 
Mises.  It  is  thus  seen  that  the  lids  are  not  wanting  in 
sensory  nerves. 

If  the  lower  portion  of  the  nose  were  cut  away  and  the 
deeper  structures  exposed  between  the  palpebral  fissure  and 
the  nosC;  we  would  find  the  lachrymal  (tear)  conducting 
apparatus,  A,  Fig.  32,  which  shows  the  canaliculi  (minute 
canals)  above  and  below  the  lakus  (small  lake),  B.  These 
empty  into  the  lachrymal  sac  (tear  sack)  C,  which  becomes 
smaller  as  it  extends  downward  toward  the  nasal  cavity  and 
is  known  as  the  lachrymal  or  nasal  duct,  D.  This  empties 
into  the  nasal  cavity  below  the  inferior  turbinate,  E,  into 
the  space  known  as  the  inferior  meatus,  F.  At  G  is  shown 
the  middle  turbinate  and  H  shows  the  nasal  cavity  proper. 
At  I  will  be  seen  the  tendo  oculi  or  palpebral  ligament  cut 


52 


THE     ANATOMY    OF    THE     EYE. 


.short.  The  lachrymal  sack  occupies  a  triangular  8i)aec  be- 
hind this  structure,  and  in  front  of  the  tensor  tarsi  or  Hor- 
ners'  muscle,  and  when  these  two  structures  are  made  taut, 
as  is  the  case  when  the  eye  is  closed,  this  arrangement  causes 
a  pulling  forward  and  outward  of  the  anterior  portion  of 
the  lachrymal  sac  by  the  palpebral  ligament,  while  at  the 


3 

c 

Jl 

/' 

'^  "iP" 

^H^ii 

___  \,|yf 

Fig.  32.    Showing  Canaliculi  and  Lachrymal   Sac  and  canal  emptying 
into  the  nasal  canal. 


same  time  the  tensor  tarsi  muscle  pulls  the  posterior  portion 
outward  and  backward,  thus  distending  the  sac.  Below  the 
lachrymal  sac  there  are  valves  in  the  lachrymal  duct  leading 
to  the  nasal  cavity.  These  open  downward  and  close  the 
duct  when  there  is  suction  from  above,  as  is  the  case  when 
the  sac  is  distending,  and  the  closing  of  the  lids  (which  has 
distended  the  sac)  has  turned  the  lachrymal  papilla,  I,  Fig. 
25,  so  that  their  tips,  where  the  lachrymal  puncta  are  located, 
are  pressed  into  the  lakus,  B,  Fig.  32,  and  C,  Fig.  25.     As 


THE    ANATOMV'    OF    THE    EYE. 


53 


the  lachrymal  duct  is  closed  there  is  produced  a  suction  at 
these  openings  so  that  any  of  the  lachrymal  fluid  (tear 
fluid)  which  may  be  in  the  lakus  is  drawn  into  the  canaliculi 
and  onward  into  the  lachrymal  sac.  When  the  eye  is  opened 
and  the  lachrymal  sac  collapses  the  valves  in  the  lachrymal 
ducts  open  and  the  fluid  is  given  free  passage  into  the  nose. 


^\  ,/ 


M 


c^ 


!*'V-,VC 


Fig.  33.    vShowing  Conjunctival  Surface  of  the  Lids. 


So  it  is  seen  that  we  have  here  a  truly  mechanical  pumping 
apparatus  to  carry  the  tears  from  the  eye.  At  J  is  seen  the 
corrugator  supracilia  muscle. 

Should  we  separate  the  lids  from  their  attachments  and 
leave  only  the  attachments  between  them  and  the  nose  and 
swing  them  around  forward,  to  clear  the  orbit,  and  look  at 
the  posterior  or  conjunctival  surface  of  the  lids,  we  would 
behold  about  the  picture  as  seen  in  Fig.  33. 

At  A  is  seen  the  lachrymal  gland  and  at  B  the  openings 
through  the  conjunctiva  where  its  ducts  empty  into  the  con- 


54  THE    ANATOMY    OF    THE    EYE. 

junctival  sac  at  the  fornix.  C  shows  the  conjunctival  tissue, 
dissected  from  the  back  of  the  Hds,  exposing  the  tarsal  carti- 
lages in  which  are  imbedded  the  meibomian  glands,  shown 
at  D,  and  their  ducts  opening  onto  the  free  margin  of  the 
lids,  E.  These  glands  secrete  a  sebaceous  (oily)  material 
which  helps  to  lubricate  the  lids  as  they  glide  over  the  eye- 
ball and  also  prevents  the  lids  from  sticking  together  when 
we  sleep.     Another  function  is  that  as  the  margins  of  the 


Fig.  34,      Showing  the  Anterior  Attachment  to  the  Eye  BaU  of  the 
Recti  Muscles. 


lids  are  kept  oiled  all  the  time,  the  tears  do  not  flow  over 
them  so  readily  and  as  the  two  lids  come  into  apposition 
at  the  outer  angle  first  and  then  gradually  close  the  pal- 
pebral fissure  from  without  inward  toward  the  nose,  the 
lachrymal  fluid  flows  inward  toward  the  lakus  instead  of 
over  the  margin  of  the  lid  and  on  to  the  cheek,  as  it  would 
do  if  it  were  not  for  this  sebaceous  material  being  so  freely 
distributed  along  the  free  margin  of  the  lid.  This  oily 
substance  also  mixes  with  the  tears  and  helps  to  prevent 
friction  between  the  eye  ball  and  lids,  as  well  as  keeping 


THE    A-N ATOMY    OF    THE    EYE.  55 

the  cornea  oiled  so  it  does  not  dry  so  quickly  as  it  other- 
wise v/ould. 

F  shows  the  location  of  the  canaliculi  and  G  the  lachrymal 
sac ;  H  shows  the  tensor  tarsi,  or  Homers'  muscle,  cut  away ; 
I  shows  the  corrugator  supracilli ;  J  shows  the  levator  labii 
superioris  et  aliqua  nasi  muscle  (the  lifter  of  the  upper  lip 
and  the  wing  of  the  nose).  This  muscle  arises  just  below 
the  inner  side  of  the  orbit. 

K  shows  the  frontal  sinus  and  L  shows  the  maxillary 
sinus.  These  two  sinuses  sometimes  become  diseased  and 
affect  the  eye  on  account  of  their  nearness  to  it. 

Should  the  lids  be  severed  throughout  their  extent  except 
at  the  inner  side  and  swung  out  across  the  nose  and  all  the 
tissue  of  the  anterior  part  of  the  orbit  dissected  away,  ex- 
cept the  globe  and  recti  muscles,  as  shown  in  Fig.  34,  we 
could  see  the  anterior  portions  and  the  attachments  of  the 
four  straight  recti  muscles.  A,  B,  C  and  D,  the  tendon  E 
and  pulley  F,  of  the  superior  oblique  and  almost  the  whole 
of  the  inferior  oblique  muscle  G  as  it  arises  from  the  floor 
of  the  orbit  well  forward  and  runs  outward  and  slightly 
backward  passing  below  the  inferior  rectus  and  is  attached  to 
the  lower  posterior  quadrant  of  the  eyeball.  H  shows  the 
ocular  conjunctiva,  cut  in  a  circle  just  outside  of  the  cornea. 

Should  we  make  a  horizontal  cross  section  through  the 
orbit  and  its  contents,  dissecting  away  all  structures  except 
the  ligaments,  fascias,  etc.,  we  would  find  the  arrangement 
about  as  shown  in  Fig.  35.  At  A  is  shown  the  lid  with  the 
orbicularis  palpebrarum  muscle  B,  and  the  tarsal  cartilage 
C,  with  the  conjunctiva  D,  lining  the  conjunctival  sac  E, 
in  which  lies  the  plica  semilunaris  Q.  At  either  side,  in  front, 
running  from  the  Hd  to  the  brim  of  the  orbital  bones,  is  seen 
the  orbito  tarsal  ligamiCnt  or  tendo  oculi  F,  and  just  back  of 
it,  at  the  internal  side,  is  found  the  tensor  tarsi  muscle  or 
Horner's  muscle  H.  Just  next  to  the  wall  of  the  orbit  and 
placed  between  the  tendo  oculi  and  the  tensor  tarsi  muscle  is 
found  the  lachrymal  sac  I.    At  either  side  of  the  globe,  run- 


56 


THE    ANATOMY    OV    THE    EYE. 


ning  forward  from  the  internal  recti  muscle  K  and  the  ex- 
ternal recti  muscle  L,  is  seen  the  check  ligaments  of  these 
muscles  G.  These  are  bands  of  fascia  from  the  muscle 
sheaths,  which  run  forward  and  blend  with  the  deep  fascia 
or  ligament  of  Lockwood,  which  stretches  across  the  front 
or  base  of  the  orbit  within  the  lids,  above  and  below 
the  palpebral  fissure.     These  check  ligaments  prevent  ex- 


'■••.Mi  '( 

A     1'.      C- 

i  ■'  JI||       . 

;fN 

./ 

^                'p 

1 

Fig.  35.    Cross  Section  of  Orbit  and  Contents. 


treme  action  of  the  muscles,  which  otherwise  might  do  harm- 
to  the  optic  nerve,  by  rotating  the  eyeball  too  greatly.  Just 
outside  of  the  posterior  portion  of  the  eyeball  is  seen  the 
space  of  Tenon  N,  which  is  a  lymph  space,  and  outside  of  it 
Tenon's  sheath  or  capsule.  Tenon's  space  is  crossed  by 
loose  bundles  of  connective  tissue,  running  from  the  sclera 
to  Tenon's  capsule  and  vice  versa.  These  are  known  as 
trabeculae  (fibrous  bands).  These  are  very  loose  and  of 
sufficient  length  to  allow  free  movements  of  the  eyeball  in 
the   socket   formed   by  Tenon's   capsule.     When   the   recti 


THE    ANATOMY    OF    THE    EYE. 


57 


muscles  come  near  to  the  eyeball,  the  sheaths  of  the  muscles 
blend  with  the  capsule  of  Tenon,  as  shown  at  J,  and  it  must 
be  borne  in  mind  that  this  connection  greatly  modifies  the 
action  of  the  recti  muscles.  Posteriorly  is  seen  the  optic 
nerve  O,  surrounded  by  the  intra  vaginal  space  P,  and  sur- 
rounding this  space  is  found  the  sheath  of  the  optic  nerve, 
which  is  continuous  with  the  sclerotic,  and  outside  of  the 


,,1 

K 

,,.  #« 

! 

r^^: 

*/' 

X       -^   H 

/            \ 

m 

■\ 

-I) 

L 

'0             J 

Fig.  36.    Vertical  Cross  Section  of  Orbit. 


Optic  nerve  sheath  is  found  the  supra  vaginal  space  S,  which 
is  continuous  with  the  space  of  Tenon.  This  is  surrounded 
by  Tenon's  capsule,  filling  in  the  spaces  between  the  eye- 
ball and  the  posterior  or  apex  of  the  orbit,  and  between  the 
muscles  and  other  structures,  is  found  the  orbital  fat  T. 
This  acts  as  a  cushion  for  the  eyeball  as  well  as  filling  the 
spaces  between  the  structures  of  the  orbit. 

Fig.  36  shows  a  vertical  cross  section  of  the  orbit ;  above 
and  in  front  is  seen  the  upper  lid  A  and  below  in  front  is 


58 


THE    ANATOMY    OF    THE    EYE. 


the  lower  lid  B,  the  slit  between  them,  the  palpebral  fissure 
C.  Back  of  the  lids,  and  in  front  of  the  cornea,  is  the  con- 
junctival sac  and  above  and  below  is  seen  the  fornices 
(folds)  D,  where  the  conjunctiva  leaves  the  lid  (palpebral 
conjunctiva)  and  folds  on  itself,  forming  the  fornix  and 
then  covering  the  anterior  of  the  eyeball  (ocular  conjunc- 
tiva), ceasing  at  the  edge  of  the  cornea.  At  E,  Fig.  36,  is 
found   the   check   ligaments   of   the    levator   palpebral    «"n- 


Fig.  37.    Showing  the  Muscles  of  the  Orbit. 


perioris  J,  and  the  iriferior  rectus  L,  and  at  F  is  seen  a  band 
of  tissue  running  from  the  upper  side  of  the  superior  rectus 
muscle  K  to  the  lower  side  of  the  levator  palpebrae.  This 
band  of  tissue  forms  the  check  ligament  of  the  superior 
rectus  riiuscle.  At  H  is  seen  the  deep  fascia  or  ligament  of 
Lockwood.  At  G  is  seen  the  inferior  oblique  muscle  with 
its  sheath  and  the  intimate  relation  of  its  sheath  with  the 


THE    ANATOMY    OF    THE    EYE. 


59 


sheath  of  the  inferior  rectus  I,  and  the  capsule  of  Tenon. 
This  is  of  importance  from  the  fact  of  the  modification  of 
the  action  of  the  inferior  obHque  which  it  causes.  At  M 
is  seen  the  orbital  fat. 

Should  the  roof  of  the  orbit  be  cut  away  and  all  the 
structures  of  the  orbit  dissected  away  except  the  muscles, 


Fig.  38.    Showing  Vessels  of  Orbit. 


eyeball  and  the  lachrymal  gland,  we  would  see  about  such 
a  picture  as  shown  by  Fig.  37.  The  levator  palpebrae  su- 
perioris  A,  which  occupies  the  uppermost  portion  of  the 
orbit,  is  cut  and  thrown  forward  and  exposes  the  superior 
rectus  B,  which  lies  just  below  it.  At  the  inner  side  and 
above  is  shown  the  superior  oblique  C,  running  through  the 
trochlea  or  pulley  D,  then  its  tendon  E  running  obliquely 
outward  and  backward  to  its  attachment  to  the  globe  F,  be- 


6o  THE    ANATOMY    OF    THE    EYE. 

neath  the  superior  rectus.  Just  beneath  and  outside  of  tlie 
superior  obHque,  is  seen  the  internal  rectus  muscle  K.  At 
A  IS  seen  the  external  rectus  muscle  and  between  it  and  the 
eyeball  is  seen  the  attachment  of  the  inferior  oblique  muscle 
H.  At  the  floor  of  the  orbit,  just  back  of  the  eyeball,  is 
shown  a  small  portion  of  the  inferior  rectus  muscle  J.  All 
these  muscles,  except  the  inferior  oblique,  arise  from  the 
ligament  of  Zinn,  which  surrounds  the  optic  foramen  at  the 
apex  of  the  orbit.  In  the  upper  anterior  portion  of  the  orbit 
is  shown  the  lachrymal  gland  I, 

Should  the  roof  of  the  orbit  be  cut  away  and  all  the 
structures  of  the  orbit  dissected  away,  except  the  arteries, 
veins,  eyeball  and  lachrymal  gland,  we  would  see  a  picture 
about  as  portrayed  in  Fig.  38.  Coming  from  the  internal 
carotid  artery,  comes  off  the  ophthalmic  artery  A,  which 
enters  the  orbit  through  the  optic  foramen  with  the  optic 
nerve.  It  first  gives  off  the  lachrymal  branch  D,  which 
takes  a  course  outward  and  upward  to  the  position  of  the 
gland  I,  which  it  supplies,  and  after  giving  off  branches  to 
the  gland,  it  pierces  the  lid  and  supplies  the  superficial 
structures  of  the  lid,  at  the  upper  outer  side  of  the  orbit. 
The  next  branches  given  off  are  the  several  short  posterior 
and  long  posterior  ciliary  arteries  B,  which  run  forward  and 
pass  into  the  eyeball  in  a  circle  around  the  optic  nerve  and 
run  forward  in  the  choroid.  Shortly  after  these  branches 
are  given  off,  the  arteria  centralis  retina  is  given  off.  This 
artery  passes  into  the  optic  nerve  ten  or  twelve  millimeters 
back  of  the  eyeball  and  passes  through  the  choroidal  fissure 
and  gives  the  blood  supply  to  the  retina.  There  are  also 
muscular  branches  given  off  which  pass  into  the  muscles  and 
run  forward  in  them  to  their  attachments  to  the  eyeball. 
These  arteries  pierce  the  sclerotic  and  enter  the  eyeball  and 
are  then  known  as  the  anterior  ciliary  arteries  C.  Then 
the  supra  orbital  branch  is  given  off,  which  runs  upward 
and  forward  and  passes  out  of  the  orbit  through  the  supra 
orbital  foramen  and  supplies  the  structures  just  above  the 


THE    ANATOMY    OF    THE    EYE.  6l 

orbit.  The  posterior  H,  and  anterior  E,  ctliemoid  branches, 
are  then  given  off.  These  pass  through  the  posterior  and 
anterior  ethmoidal  foramen,  which  are  found  in  the  upper 
posterior  portion  of  the  internal  bony  wall  of  the  orbit. 
They  first  pass  into  the  cranial  cavity,  then  run  downward 
through  the  cribriform  plate  of  the  ethmoid  bone  to  sup- 
ply the  internal  and  anterior  portion  of  the  nose.  Ante- 
riorly the  ophthalmic  artery  gives  off  the  frontal  artery. 
These  two  then  pierce  the  lids  and  one  or  the  other  forms 
an  anastomosis  with  the  angular  artery,  which  is  the  termi- 
nal branch  of  the  facial  artery.  This  is  of  importance, 
from  the  fact  that  if  the  internal  carotid  artery  or  the 
posterior  portion  of  the  ophthalmic  artery  should  be  oc- 
cluded (stopped  up),  collateral  circulation  would  be  estab- 
lished by  this  route.  Accompanying  all  the  larger  arteries 
are  found  the  veins,  which  carry  the  return  flow  of  blood, 
and  these  veins  are  known  by  the  sam.e  name  as  the  artery 
which  they  accompany.  However,  there  are  no  veins  leav- 
ing the  eyeball  with  the  posterior  ciliary  arteries,  but  the 
drainage  from  the  choroid  is  by  the  Vena  Vorticosse,  J. 
These  leave  the  eyeball  just  back  of  the  equator  and  there 
are  usually  about  five  in  number.  All  these  veins  join  to 
form  the  ophthalmic  vein  L,  which  passes  backward  through 
the  sphenoidal  fissure  and  empties  into  the  cavernous  sinus. 
As  shown  at  B,  Fig.  38,  the  ophthalmic  artery  gives  off 
several  small  branches  which  enter  the  eyeball  in  a  circle 
around  the  optic  nerve.  There  are  some  twelve  to  twenty 
of  these,  which  are  known  as  the  short  posterior  ciliary 
arteries  and  two  known  as  the  long  posterior  ciliary  arteries. 
Should  we  enucleate  the  eyeball  and  dissect  away  all  the 
tissues  down  to  the  choroid  and  leave  only  the  long  and 
short  ciliary  arteries.  Fig.  39  would  give  us  a  fair  repre- 
sentation of  their  distribution.  The  short  posterior  ciliary 
arteries.  A,  from  twelve  to  twenty  in  number,  enter  the  eye- 
ball by  piercing  the  sclerotic  in  a  circle  just  outside  of  the 


62 


THE    ANATOMY    OF    THE    EYE. 


optic  nerve.  Immediately  after  entering  the  sclerotic,  they 
divide,  the  main  portion  running  forward  (See  P,  Fig.  23) 
and  enter  the  choroid,  breaking  up  into  smaller  vessels  and 
lay  in  three  strata  or  layers,  the  layer  of  large  blood  ves- 
sels, the  layer  of  small  blood  vessels,  which  is  immediately 


Fig.  39.     Showing  Ciliary  Arteries. 


beneath  it,  and  the  chorio  capillaris  or  capillary  layer,  which 
is  the  innermost  layer  and  is  just  beneath  the  retina.  The 
larger  vessels  run  forward  in  the  choroid  and  ciliary  proc- 
esses to  the  ciliary  bodies,  which  are  just  inside  of  the 
ciliary  muscles  B,  where  they  end  in  capillary  loops  and 
turn  back  as  venous  capillaries,  while  the  branches 
given  off  in  their  course  form  the  layer  of  smaller  blood  ves- 
sels and  these  again  break  up  into  the  chorio  capillaris. 
The  branches  that  turn  toward  the  optic  nerve,  just  after  the 
short  posterior  ciliary  arteries  enter  the  sclerotic  (See  Fig. 


THE    ANATOMY    OF    THE    EYE.  63 

23)  form  ii  circle  of  anastomosis  around  the  optic  nerve, 
known  as  circulus  of  Zinn,  as  shown  at  O,  Fig.  23.  This 
circle  furnishes  a  copious  blood  supply  to  the  head  of  the 
optic  nerve  as  well  as  furnishing  a  path  for  the  establish- 
ment of  collateral  circulation,  when  there  is  trouble  with  the 
branches  which  supply  the  nerve  sheath  with  which  they  also 
connect  or  anastomose. 

There  are  two  long  posterior  ciliary  arteries,  C,  which 
enter  the  eyeball  a  little  farther  out  than  the  short  posterior 
ciliary  arteries,  one  to  the  outer  side  of  the  nerve  and  one 
to  the  inner  side.  These  run  forward  clear  to  the  ciliary 
region,  before  they  branch,  and  then  when  they  do  branch 
they  join  with,  or  anastomose  with  the  anterior  ciliary  ar- 
teries, which  enter  the  eyeball  at  the  attachments  of  the  recti 
muscles  D,  and  these  then  form  what  is  known  as  the  cir- 
culus major  (larger)  of  the  Iris  E,  Fig.  39,  and  2,  Fig.  23. 
From  this  circle  is  given  off  the  vessels  for  the  iris,  which 
run  radially  in  toward  the  pupil  G,  and  when  these  come 
near  to  the  free  margin  of  the  iris  another  circle  of  an- 
astomosis is  formed,  which  is  known  as  the  circulus  minor 
F,  Fig.  39,  and  E,  Fig.  23.  Inside  of  this,  toward  the 
pupil,  are  given  off  arterial  capillaries  which  turn  back  as 
veins,  which  are  drained  by  the  anterior  ciliary  veins,  which 
leave  the  eyeball  at  the  muscular  attachments  D.  At  H  is 
seen  the  vena  vorticosa  (whirlpool)  and  at  I  is  seen  the  optic 
nerve. 

Should  the  eyeball  be  enucleated  and  the  sclerotic  and  the 
tissues  dissected  oft',  leaving  only  the  veins  of  the  posterior 
four-fifths  of  the  eyeball,  we  would  find  practically  the 
arrangement  as  seen  in  Fig.  40.  The  smaller  veins  pass 
back  from  the  ciliary  bodies  at  A  from  underneath  the 
ciliary  muscle  F.  These  veins  constantly  join  or  anastomose 
with  others  and  form  four  or  five  whirls,  B,  finally  join  to 
form  the  four  or  five  vena  vorticosae  (whirlpool  veins)  C, 
which  leave  the  eyeball  just  back  of  the  equator  and  empty 
into  the  ophthalmic  vein.     See  J.  Fig.  38. 


64 


THE    ANATOMY    OF    THE    KYE. 


As  previously  mentioned  the  ophthalmic  artery  gives  off 
one  branch,  which  enters  the  optic  nerve  at  its  under  sur- 
face and  about  ten  to  tv^elve  millimeters  back  of  the  eye- 
ball>  which  is  known  as  the  arteria  centralis  retina  (central 


Fig. 40.     Veins  of  the  EyebaU 


artery  of  the  retina),  from  the  fact  that  it  enters  the  eye 
ball  at  the  optic  disc  and  spreads  out  to  supply  the  retina 
(See  3,  Fig.  23),  and  if  we  should  take  an  eyeball  and  make 
a  coronal  cut  down  through  it  at  the  equator,  then  hold  it 
up  and  look  at  the  inner  surface  of  the  globe,  we  would 
see  the  picture  as  portrayed  in  Fig.  41.  At  the  disc  A  are 
seen  the  arteries  emerging  from  the  head  of  optic  nerve  or 
disc  and  the  veins  leaving.  The  artery  first  breaks  up  into  two 
branches,  one  running  upward,  the  other  downward.  These 
are  known  as  the  upper,  B,  and  lower,  C,  branches.  These 
in  turn  each  divide  into  two  branches.  Each  of  these  four 
branches  runs  obliquely  outward  from  the  disc,  the  upper  one 


THE    ANATOMY    OP^    THE    EYE. 


65 


running  inward  toward  the  nose  is  called  the  superior 
nasal,  D,  and  the  one  running  upward  and  outward  and 
toward  the  temple  is  called  the  superior  temporal,  E,  while 
the  one  below,  running  inward  toward  the  nose  is  known  as 
the  inferior  nasal,  F,  and  the  one  running  downward  and 


E        ^^ 

/« 

'"^ 

i — '- 

Fig.  41.    Arteries  of  the  Retina 


outward  toward  the  temple  is  called  the  inferior  temporal, 
G. '  The  farther  divisions  of  these  arteries  are  unnamed. 
However,  there  are  usually  one  or  two  small  arteries,  which 
run  from  the  disc  toward  the  maculae,  which  when  present 
arc  called  the  macular  arteries,  H.  These  arteries  and 
veins  lie  in  the  retina,  I,  and  the  arteria  centralis  retina  is 
what  is  known  as  a  terminal  artery,  or  in  other  words,  it 
forms  no  anastomosis  with  any  other  set  of  arteries,  conse- 
quently when  it  breaks  up  into  capillaries,  these  turn  back  as 
veins.  These  keep  joining  together  and  get  larger  and 
larger  until  there  are  large  veins  formed,  which  are  named 
the  same  as  the  arteries  which  they  accompany.    As  there  is 


66  THE    ANATOMY    OF    THE    EYE. 

usually  a  vein  accompanying  each  artery,  these  join  at  the 
disc  and  form  the  vena  centralis  retina  which  leaves  the 
eyeball  within  the  optic  nerve  and  lies  within  it  for  some  ten 
or  twelve  millimeters.  It  then  leaves  the  nerve  and  empties 
into  the  ophthalmic  vein  (See  Fig.  23).  The  fact  of  the 
arteria  centralis  retina  being  a  terminal  artery  in  the  retina 
having  no  collateral  loops  or  anastomosis,  as  is  the  case  in 
almost  all  other  portions  of  the  body,  makes  this  of  especial 
clinical  significance,  for  if  it  becomes  occluded,  the  nourish- 
ment is  cut  off  from  the  retina  and  sight  is  lost  and  the 
retina  atrophies  in  an  exceedingly  short  period. 

Just  to  the  temporal  side  of  the  disc  is  seen  the  macula 
(spot)  and  at  its  center  the  fovea  centralis  (central 
spot)  J.  It  is  so  named  from  the  fact  that  it  is  the  thinnest 
spot  in  the  whole  retina  and  turns  yellow  after  death.  It  is 
not  seen  as  a  yellow  spot  during  life,  with  an  ophthalmo- 
scope, as  some  inexperienced  ones  think,  but  as  a  dark  area 
devoid  of  visible  blood  vessels  and  the  yellow  appearance 
which  we  see  in  examining  the  posterior  inner  surface  of  the 
eyeball  after  death  is  a  post  mortem  (after  death)  change. 
K  shows  the  choroid  and  L  the  scleral  coat  of  the  eyeball. 

Should  we  cut  away  the  roof  of  the  orbit  and  dissect 
away  all  the  tissues  except  the  nerves,  eyeball,  recti  muscles, 
levator  superioris  and  the  lachrymal  gland,  Fig.  42  would 
be  a  fair  representation  of  what  we  would  observe.  At  A 
we  see  the  sixth  cranial  or  abduceus  nerve,  which  innervat-es 
the  external  rectus  muscle  J,  and  at  B  is  seen  the  third 
cranial  nerve  or  the  motor  oculi,  which  furnishes  nerve  im- 
pulse to  the  levator  palpebrae  superioris  K,  the  superior 
rectus  L,  the  internal  rectus  M,  the  inferior  rectus  N,  and 
the  inferior  oblique  O,  besides  giving  branches  to  the  ciliary 
or  lenticular  ganglion  Q.  At  C  is  shown  the  fourth  cranial 
or  patheticus  (cry)  nerve  which  supplies  the  superior 
oblique  muscle  I.  At  D  is  shown  the  fifth  cranial,  trigem- 
inus or  trifacial  nerve,  and  E  the  gasserian  ganglion  on  the 
fifth  nerve,  and  at  F  the  upper  or  ophthalmic  branch  of  the 


THE    ANATOMY    OF    THE    EYE. 


67 


fifth  nerve  which  supplies  sensation  to  the  orbit,  eyeball  and 
its  structures  as  well  as  the  lids,  and  G  the  superior  maxil- 
lary nerve  or  the  middle  branch  of  the  trifacial  or  fifth 
nerve,  and  H  the  lower  branch  or  the  inferior  maxillary 
nerve.  However,  we  are  only  particularly  interested  in  the 
first,  upper  or  ophthalmic  branch,  and  just  slightly  inter- 
ested in  the  second,  or  superior  maxillary  branch,  for  the 
ophthalmic  nerve  gives  off  first  the  nasal  branch,  R,  which 


Fig.  42.    The  Nerves  of  the  Orbit  from  Above. 


runs  upward  and  inward  through  the  orbit,  giving  a  branch 
or  root  S  to  the  lenticular  ganglion  Q,  then  passes  out  of 
the  orbit  to  re-enter  the  cranial  cavity  through  the  ethmoid- 
al foramen,  then  it  leaves  the  cranial  cavity  again  through 
the  cribiform  plate  of  the  ethmoid  bone  and  supplies  sensa- 
tion to  the  anterior  portion  and  the  tip  of  the  nose,  and  it  is 
this  branch  which  accounts  for  the  reflexes  between  the 
nose  and  the  eye.  Then  the  ophthalmic  gives  off  the  lachry- 
mal branch,   T,   which   runs   upward   and  outward   to  the 


68 


THE    ANATOMY    OF    THE    EYE. 


lachrymal  gland,  I  J,  and  after  supplying  the  gland  it  pierces 
the  lid  and  supplies  sensation  to  the  upper  outer  part  of  the 
lid  (See  B,  Fig.  31).  The  ophthalmic  then  gives  one  or 
two  branches  or  roots  to  the  lenticular  ganglion  direct  and 
continues  upward  and  forward.  The  main  portion  of  the 
nerve  leaves  the  orbit  through  the  supra  orbital  foramen 
and  is  known  as  the  supraorbital  nerve  (See  A,  Fig.  31). 
However,  just  before  leaving  the  orbit  it  gives  oflf  a  branch 
which   divides,  and  one  branch  pierces  the  lid  above  the 


Fig.  43.    The  Nerves  of  the  Orbit  from  the  side. 


pulley  or  troclea,  V,  of  the  superior  oblique  muscle.  This 
branch  is  known  as  the  supra  trochlear  (See  E,  Fig.  31)  ; 
the  other  one  pierces  the  lid  just  below  the  trochlea  and  is 
known  as  the  infra  trochlear  branch  (See  E,  Fig.  31). 

Should  we  make  a  vertical  section  of  the  walls  of  the 
orbit  and  dissect  away  the  tissues,  leaving  only  the  eyeball, 
nerves  and  lachrymal  gland,  we  would  have  the  appearance 
as  shown  in  Fig.  43.  At  A  is  shown  the  sixth  or  abduceus 
nerve,  which  is  cut  and  thrown  up  at  I,  and  at  B  is  seen  the 
third  cranial  or  motor  oculi  nerve,  and  at  J  its  branches  or 
roots  to  the  lenticular  ganglion,  K,  and  at  D  is  shown  the 


THE    AN  ATOM  V    OF    THE    EYE.  69 

fifth  cranial  nerve,  and  at  E  the  gasserian  ganglion.  At  F 
is  shown  the  first  division,  which  is  known  as  the  ophthalmic 
nerve,  and  at  L  is  shown  its  branches  to  the  lenticular 
ganglion.  At  G  is  shown  the  second  division  or  superior 
maxillary  nerve,  but  in  the  study  of  the  eye  we  are  only 
interested  in  two  of  its  branches ;  first  the  one  shown  at  M, 
known  as  the  orbital  nerve,  which  goes  to  the  lower  outer 
side  of  the  eyeball,  forming  an  anastomosis  with  the  lachry- 
mal nerve,  T,  and  the  terminal  branch  runs  forward  and 
passes  out  onto  the  face  through  the  infra  orbital  foramen 
(See  D,  Fig.  31)  and  supplies  the  sensation  to  the  lower  lid 
and  region  just  below  the  eye.  This  branch  is  known  as  the 
infra  orbital  nerve. 

The  lenticular  ganglion,  K,  is  of  vast  importance  to  the 
eyeball.  It  is  a  small  pinkish  body  about  the  size  of  a  pin- 
head  and  is  situated  some  seven  to  ten  millimeters  back  of 
the  eyeball.  On  the  outer  side  of  the  optic  nerve,  between 
it  and  the  ophthalmic  artery,  it  receives  filaments,  or  roots, 
J,  from  the  motor  oculi  nerve,,  which  are  motor  from  the 
nasal  nerve,  L,  as  well  as  from  the  ophthalmic  nerves  which 
are  sensory.  It  also  receives  filaments  or  roots  from  the 
sympathetic  nervous  system,  which  comes  from  the  carotid 
plexus.  Thus  it  is  seen,  there  are  motor,  sensory  and  sym- 
pathetic filaments  received  by  it.  Then  from  this  ganglion 
is  given  oflf  the  posterior  ciliary  nerves,  N.  These  are  mixed 
nerves  and  carry  motor,  sensory  and  sympathetic  fibers. 
These  nerves,  from  twelve  to  twenty  in  number,  enter  the 
eyeball  posteriorly  with  the  posterior  ciliary  arteries  (See 
A,  Fig.  39,  and  A,  Fig.  44).  These  pierce  the  sclerotic  just 
outside  of  the  optic  nerve  in  a  circle  and  pass  forward  most- 
ly in  the  supra  choroidal  space,  and  if  we  should  enucleate 
an  eyeball  and  dissect  away  the  sclerotic  and  all  other  struc- 
tures except  the  nerves,  we  would  have  a  picture  as  shown 
in  Fig.  44.  The  posterior  ciliary  nerves,  ?»,  run  forward  in 
the  supra  choroidal  space  and  give  numerous  branches  to  the 
choroid,  C,  in  their  course.     They  then  break  up  into  small 


7o 


TH£    ANATOMY    Ol^    THE    EYE. 


branches,  D,  and  these  form  a  plexus  in  the  ciliary  muscle, 
E,  and  from  thif  plexus  is  given  off  branches  to  the  ciliary 
muscle  which  are  motor  to  the  ciliary  bodies  which  are  sym- 
pathetic and  sensory,  then  other  branches  to  the  iris,  F, 
which  are  sensory,  motor  and  sympathetic,  the  motor  for 
the  spincter  pupillae  (See  K,  Fig.  23),  sympathetic,  for  the 
dilator  pupillae  muscle.    Other  branches  go  from  the  ciliary 


Fiff.  44.    Showing  Ciliary  Nerves. 

plexus  to  the  cornea  which  are  entirely  sensory.  Thus  it 
will  be  seen  that  the  nerve  supply  to  the  eye  is  abundant  and 
of  all  three  varieties,  motor,  sensory  and  sympathetic. 

Having  covered  the  gross  anatomy  of  the  eye  pretty  thor- 
oughly, we  will  now  pass  to  the  more  minute  anatomy  or 
Histology  and  in  so  doing  it  is  well  for  the  reader  to  be 
familiar  with  the  gross  anatomy,  in  order  to  be  familiar  with 
the  relation  of  parts. 


CHAPTER  III. 

HISTOLOGY. 

We  will  first  take  up  the  lids  or  palpebrae  (from  palpare — 
to  stroke).  These  are  two  crescentic  folds,  which  grow 
from  above  down  and  from  below  upward  and  cover  the 
front  of  the  eyeball.  See  Figs.  9  to  17,  showing  their  de- 
velopment from  the  margin  of  the  orbit.  Their  function  is 
purely  the  protection  of  the  eyeball  and  they  contain  many 
glands,  all  of  which  secrete  substances  which  play  their  parts 
in  the  physiological  functions  of  the  lids.  The  lids  also  con- 
tain two  semilunar  plates  with  their  convex  border  turned 
away  from  the  palpebral  slit.  These  are  very  dense,  fibrous 
plates,  known  as  the  tarsal  cartillages  U,  Fig.  45.  How- 
ever, they  have  nothing  in  common  with  cartillage,  except 
their  density,  they  being  made  up  wholly  of  white  fibrous 
tissue.  However,  they  were  named  by  the  ancient  anato- 
mists prior  to  the  time  of  our  ability,  by  chemical  analysis, 
to  determine  accurately  the  constituents  of  all  tissues  and 
bodies. 

The  outer  or  anterior  surface  is  covered  with  epithelium 
while  the  inner  or  posterior  surface  is  covered  with  mucous 
membrane,  the  epithelium  changing  its  nature  at  the  free 
margin  of  the  lid. 

Fig.  45  shows  a  vertical  cross  section  of  the  upper  lid. 
At  A  is  shown  the  epithelium ;  at  B  is  shown  the  hair  follicles 
of  the  small  white  hairs,  which  are  scattered  over  the  ante- 
rior surface  of  the  lids.  At  C  is  shown  the  sweat  glands ; 
at  D  the  subepithelial  tissue,  or  areolar  tissue,  which  dif- 
fers somewhat  from  that  found  in  other  parts  of  the  body, 
from  the  fact  that  fat  is  not  readily  deposited  in  it,  as  is  the 
case  elsewhere  in  the  body.  Lying  immediately  below  the  are- 
olar tissue  is  found  the  orbicularis  palpebrarum  muscle.  The 
cross  sections  of  the  bundles  are  seen  at  E  (also  see  Fig.  27) 

71 


72 


THE    ANATOMY    OF    THE    EYE. 


and  at  F  are  shown  the  hair  folHcles  of  the  ciha  or  lashes. 
At  G  are  seen  the  modified  sweat  glands  of  Moll  and  at  H 
are  .shown  the  sebaceous  glands  connected  with  the  lash  or 
cilia  in  the  lids.  These  glands  are  known  as  Zeisse's  glands. 
At  I  is  seen  the  muscle  of  Riolanis.  This  is  the  involun- 
tary muscle  for  closing  the  eye ;  it  also  re-enforces  the  orbicu- 


Fig.  45.    Vertical  Cross  Section  of  the  Upper  EyeUd. 

lars  and  brings  the  margins  of  the  lids  into  close  and  firm 
apposition.  J  points  to  the  region  where  the  epithelium 
changes  its  nature  to  that  of  a  mucous  membrane,  such  as 
lines  all  cavities  or  internal  openings  which  communicate 
with  the  external  world,  and  in  this  case  is  known  as  the 
conjunctiva.  At  K  is  shown  one  of  the  ducts  of  a  Mei- 
bomian Gland,  and  L  shows  the  secreting  portion  of  the 


THE    AN  ATOM  V    OF    THE    EYE. 


73 


gland,  which  is  imbedded  in  the  tarsal  cartillage.  M  shows 
the  palpebral  conjunctiva,  and  at  N  is  seen  a  cross-section 
of  one  of  the  superior  palpebral  arteries  (see  G  and  J,  Fig. 
29).  There  is  a  free  anastomosis  between  these  arteries  and 
those  of  the  inner  or  conjunctival  surface  formed  by  numer- 
ous arteries  piercing  the  tarsal  cartillage.  At  O  are  seen  the 
post  tarsal  papillae,  which  are  folds,  and  the  depressions  be- 


Fig.  46.    Eyelid  Showing  Portion  of  a  Hair  FoUicle. 


tween  them  are  called  Henly's  glands.  At  P  are  shown  the 
glands  of  Waldeyer.  At  Q  is  shown  the  involuntary  muscle 
of  Mueller,  and  it  is  this  muscular  bundle  which  opens  the 
eye  involuntarily.  At  R  are  seen  Kraus'  glands,  just  above 
the  fornix  (arch)  of  the  conjunctiva.  S  shows  the  fibers 
of  the  levator  palpebrae  superioris  (see  B,  Fig.  28),  and  at 
T  are  shown  the  fibers  from  this  muscle,  which  pass  outward 


74 


THE    ANATOMY    OF    THE    EYE. 


between  the  fibers  of  the  orbicularis  palpebrarum,  and  are 
attached  to  the  skin  as  shown  at  A  in  Fig.  52. 

This  fasciculus  is  a  wise  provision  of  nature,  for  when 
the  lid  is  raised  it  keeps  the  skin  taut  between  its  attachment 
and  the  free  margin  of  the  lid,  and  draws  it  up  with  the  lid, 
while  the  skin  above  drops  down  over  it,  making  a  fold 


Fig.  47.    Showing  Zeisse's  Glands,  Modified  Sweat  Glands  of  Moll  and 
Meibomian  Glands. 

in  the  skin  at  about  the  middle  of  the  lid,  and  in  this  way 
takes  care  of  the  loose  skin  when  the  lid  is  raised,  other- 
wise it  would  drop  down  over  the  edge  of  the  lid  and  inter- 
fere with  vision. 

Fig.  46  shows  the  innermost  portion  of  a  hair  follicle; 
G  the  papillae  in  the  follicle,  from  which  the  hair  grows 
and  receives  its  nourishment  mainly,  and  H  the  cup  in  the 
end  of  the  hair  shaft. 


THE    ANATOMV    OF    THE    EYE.  75 

Fig.  47  also  shows  a  hair  follicle.  B,  Figs.  46  and  47, 
shows  the  sebaceous  gland,  known  as  Zeisses  glands.  In 
this  location,  these  glands  are  compound  sacular  glands, 
the  sacks  filled  with  secreting  cells,  which  secrete  an  oily 
material  called  sebum,  which  is  poured  into  the  hair  follicle 
and  travels  along  the  lashes  and  keeps  them  oiled,  so  they 
are  always  soft  and  pliable.  C,  Fig.  46  and  47,  shows  the 
modified  sweat  glands  of  Moll,  which  are  tubular  glands, 
lined  with  secreting  cells,  which  in  other  parts  of  the  body 
lie  doubled  up  in  knots  in  the  areolar  tissue  with  straight 
tubes  running  to  the  surface.  These  modified  sweat  glands  lie 
in  the  muscle  of  Riolanis,  just  back  of  the  lashes.  The  modi- 
fication of  these  glands  on  the  margin  of  the  lids  is  due  to  the 
fact  that  instead  of  opening  onto  the  surface  as  sweat  glands 
do  elsewhere  on  the  body,  these  empty  into  the  hair  follicle 
and  this  watery  secretion  becomes  mixed  with  the  sebum  from 
Zeisse's  glands  and  thereby  renders  it  more  viscid  or  watery. 
This  serves  the  purpose  of  keeping  the  lashes  constantly 
covered  with  this  thin,  viscid,  oily  substance,  which  facilitates 
their  capacity  for  catching  dust,  thereby  increasing  the  use- 
fulness of  the  lashes  in  protecting  the  cornea  against  dust. 
F,  Fig.  46  and  47,  shows  the  muscle  of  Riolanis,  which  is  a 
small  muscular  bundle,  which  surrounds  the  palpebral  fis- 
sure and  arises  from  the  tendo  oculi  (see  A  and  B,  Fig.  26), 
however  it  is  really  a  part  of  the  orbicularis  palpebrarum 
and  is  the  involuntary  muscle  to  close  the  eye  when  the  cor- 
nea becomes  dry.  When  acting  in  conjunction  with  the  orbic- 
ularis in  closing  the  eye,  it  causes  a  folding  of  the  free  mar- 
gin of  the  lid  and  reinforces  the  orbicularis  and  brings  the 
lid  margins  more  closely  together.  D,  Fig.  46  and  47,  shows 
a  duct  of  one  of  the  meibomian  glands  and  at  E,  Fig.  46 
and  47,  are  the  gland  cells,  which  secrete  the  sebaceous 
material  which  is  poured  out  on  the  free  margin  of  the  lid. 
The  meibomian  glands  are  modified  sebaceous  glands,  being 
tubular  with  many  blind  pouches  or  sacks  connecting,  filled 
with  secreting  cells.  There  arc  from  twenty  to  thirty  of 
these  glands  in  each  lid.  They  are  imbedded  in 
the    conjunctival    surface    of    the    tarsal    cartillage  and 


76 


THE    ANATOMY    OF    THE    EYE. 


arc  readily  seen  in  the  human  Hd  (when  inverted)  as 
white  Hnes,  and  their  openings  are  readily  seen  on  the  free 
margin  of  the  lid.  See  Figs.  33  and  34.  This  secre- 
tion renders  four  important  services  to  the  eye :     First,  this 


■ 

■ 

^ 

^H 

J 

'■  "^^  rj^HBBB^^^^I 

Fig.  48.    Showing  the  Tarsal  PapiUae. 


oily  substance  prevents  the  lids  from  sticking  together  when 
we  sleep ;  second,  it  keeps  the  margins  of  the  lids  oiled  and 
prevents  the  tears  from  flowing  over  their  edges,  when  the 
eyelids  are  being  closed,  for  it  will  be  remembered  that  the 
lids  come  into  apposition  at  the  outer  canthus  first,  then  the 
slit  is  gradually  closed  from  without  inward  and  any  tears 
which  have  accunuilated  in  the  palpebral  fissure  flow  along  in 


THE    ANATOMY    OF    THE    EYE. 


77 


front  of  the  closing  edges;  thus  they  arc  directed  into  the 
lakus  (see  C,  Fig.  25)  ;  third,  it  keeps  the  cornea  oiled,  which 
prevents  the  cornea  from  dessication  or  drying  so  readily ; 
fourth,  its  mixing  with  the  tears  and  keeping  the  conjunc- 
tival sac  so  freely  lubricated,  prevents  friction  of  the  struct- 
ures as  they  glide  over  each  other  in  the  opening  and  closing 
of  the  eye  lids. 


Fig.  49.    Showing  Henle's  Glands. 


A,  Fig.  48,  shows  the  post  tarsal  papillae,  which  in  reality 
are  only  folds  or  Rouga  of  the  conjunctiva  which  have  the 
appearance  of  being  small  elevations  when  seen  in  cross  sec- 
tion and  where  the  mucous  surfaces  are  brought  into  close 
proximity,  as  is  the  case  in  the  furrows  or  depressions,  the 
cells  change  their  nature,  and  we  find  these  furrows  lined 
with  columnar  epithelial  cells  as  shown  at  A  in  Fig.  49, 
and  they  are  called  Henle's  glands.  These  glands,  or  folds, 


78 


THE    ANATOMY    OF    THE    EYE. 


become  more  marked  as  age  advances.     These  surfaces  con- 
tain many  goblet  cells  and  secrete  more  or  less  mucus. 

At  A,  Fig.  50,  are  seen  the  glands  of  Waldeyer.  These 
glands  are  in  the  nature  of  sweat  glands  and  they  with 
Kraus'  glands  (A,  Fig.  51)  secrete  the  tears  under  ordinary 
circumstances.  At  B,  Fig.  51,  are  shown  cross  sections  of 
the  lachrymal  gland,  which  is  a  compound  tubulo  racemose 
gland  resembling  serous  or  fluid  secreting  glands  in  other 


Fig.  50. 


parts  of  the  body.  This  is  a  rather  larger  gland  than  any 
we  have  seen  in  the  lid  before.  It  is  located  in  the  upper 
lid,  at  the  upper  outer  side  of  the  orbit  (see  E,  Fig.  28)  ;  it 
is  almond-shaped  and  the  size  of  a  small  almond  kernel. 
The  secretions  reach  the  conjunctival  sac  by  some  ten  or 
twelve  ducts  (C,  Fig.  51),  which  empty  into  the  fornix 
(arch)  of  the  conjunctiva.  D,  Fig.  51.  The  lachrymal  gland, 
only  pours  forth  its  secretions  when  the  eye  is  irritated, 
and  this  washes  or  floods  the  conjunctival  sac  quite  freely. 


THE    ANATOMY    OF    THE    EYE. 


79 


as  when  the  eye  Is  irritated  by  a  foreign  body  or  when  we 
cry,  and  the  secretion  of  tears  is  so  copious,  that  our  lachry- 
mal apparatus  cannot  carry  away  all  the  fluids,  and  we  find 
the  tears  flowing  over  the  lower  lid  onto  the  cheek  at  such 
times. 

The  conjunctiva  (joined  together)  is  the  mucous  mem- 
brane which  lines  the  conjunctival  sac  (the  joined  sac), 
which  is  really  two  crescentic  culdesacs,  one  between  each 


Fig.  51. 

eyelid  and  the  eyeball,  and  they  are  really  separated  by  the 
palpebral  fissure,  when  the  lids  are  open.  However,  it  is 
a  complete  oval  sac  when  the  lids  are  closed.  This  mucous 
membrane  commences  at  the  free  margin  of  the  lid, 
at  B,  Fig.  52,  by  the  transformation  of  the  epithelium 
into  mucous  epithelium,  the  arrangement  of  the  cells  is  the 
same  as  in  the  epithelium  in  other  parts  of  the  body,  the 
outermost  cells  being  squamous  (scaly),  the  middle  cells 
being  irregularly  round  or  polyhedral  (many  sided)  cells, 
while  the  innermost  are  columnar  (long)  cells.     These  lie 


8o 


THE    ANATOMY    OF    THE    EYE. 


on  a  loose  membrane  which  is  well  supplied  with  blood 
vessels,  and  the  tissue  being  loose  and  transparent, 
it  gives  a  free  flow  of  Lymph.  That  part  of  the 
conjunctiva  lining  the  posterior  surface  of  the  lids 
is  known  as  the  palpebral  conjunctiva  (C,  Fig.  52). 
When  it  reaches  well  back  under  the  lids,  it  folds 
on    itself  and    becomes    adherent    to    the    sclerotic     (H, 


Fig  52. 


Fig.  52).  This  fold  is  called  the  fornix  conjunctiva  (D, 
Fig.  52).  The  portion  of  the  conjunctiva  which  covers  the 
eyeball  (E,  Fig.  52)  is  called  the  ocular  or  bulbar  con- 
junctiva. The  ocular  conjunctiva  is  transparent  and  through 
it  we  can  see  the  sclera,  which  is  opaque  and  white.  It  is 
freely  movable  over  the  sclerotic  (K,  Fig.  52)  and  by  ma- 
nipulation we  can  see  the  blood  vessels  of  the  conjunctiva 
(H,  Fig.  52)  change  their  position,  while  the  blood  vessels 


THE    ANATOMY    OF    THE    EYE. 


8l 


of  the  sclera,  which  are  deeper  set  (J,  Fig.  52),  remain 
stationary.  When  the  conjunctiva  reaches  the  outer  margin 
of  the  cornea  (G,  Fig.  52)  the  basement  tissue  ends,  but 
the  epithelium  continues  over  the  front  of  the  cornea  (F, 
Fig.  52)  and  forms  the  anterior  or  stratified  epithelial  layer 
of  the  cornea,  and  is  called  the  conjunctival  portion  of  the 


H   G  JF  /^  E 


Fig.  53. 


cornea.  The  blood  vessels  of  the  conjunctiva  end  at  the 
corneal  margin  in  a  circle  of  capillary  loops  (I,  Fig.  52,  and 
F,  Fig.  54),  very  superficially  placed. 

The  cornea  (horn-like)  A,  Fig.  23,  and  M,  Fig.  25, 
forms  about  the  anterior  sixth  of  the  eyeball.  It  is  a  highly 
transparent  structure,  allowing  the  light  from  the  external 
world  to  enter  the  eyeball,  and  is  the  first  of  the  refractive 


82  THE    ANATOMY    OF    THE    EYE. 

media  through  which  this  Hght  passes  on  its  way  to  the 
retina.  It  is  made  up  of  five  layers,  as  shown  in  cross  sec- 
tion in  Fig.  53 ;  A,  the  anterior  stratified  epithehal  layer ; 
B,  Bowman's  membrane,  or  the  anterior  homogeneous  mem- 
brane; C,  the  lamina  propria  (proper  layer)  ;  D,  Decimet's 
membrane  or  the  posterior  homogeneous  layer,  and  E,  the 
endothelial  layer ;  the  latter  lining  the  cornea  on  its  surface 
bounding  the  anterior  chamber.     See  S,  Fig.  23. 

The  anterior  stratified  epithelium,  as  before  stated,  is  con- 
tinuous with  the  epithelial  layer  of  the  conjunctiva.  As  its 
name  implies,  its  cells  differ  at  different  depths.  The  outer- 
most, F,  is  made  of  squamous  (scaly)  cells,  G  is  formed  by 
hexagonal  (many-sided)  cells,  and  the  innermost  layer,  H, 
is  formed  by  Columnar  (long,  square)  epithelial  cells  and 
this  is  the  layer  where  all  new  cells  are  formed  by  the  divi- 
sion and  growth  of  these  columnar  cells,  and  as  new  cells 
are  formed  the  older  ones  are  pushed  outward  toward  the 
surface  and  become  hexagonal,  and  as  this  process  continues, 
the  cells  are  pushed  farther  and  farther  out.  They  lose 
their  nuclei  and  become  mere  flat  scales  and  finally  lose  their 
adhesive  qualities  and  are  disquamated  (thrown  off)  and 
wash  away  with  the  tears.  These  cells  are  held  together 
both  from  the  cement  substance  lying  between  them  and  by 
the  little  projections  from  the  surface  of  the  cells  them- 
selves. When  these  projections  are  found  on  a  cell,  they 
are  called  prickle  cells,  and  this  is  the  nature  of  these  cells 
in  the  lower  or  inner  layers. 

Passing  from  without  inward,  the  next  layer,  B,  is  Bow- 
man's membrane,  or  the  anterior  homogeneous  lamina. 

As  the  name,  homogeneous  lamina,  implies,  this  layer 
when  seen  with  the  microscope  reveals  no  structural  frame 
work,  but  appears  as  a  solid  gelatinous  layer.  However,  if 
this  tissue  be  macerated  (soaked)  in  an  alkaline  solution  and 
the  cement  substance  dissolved  out,  it  will  be  found  to  be 
formed  of  connective  tissue  bundles.  This  layer  ends  at 
the   periphery  of   the   cornea.     The  next   layer,   C,   is  the 


THE    ANATOMY    OF    THE    EYE. 


83 


Lamina  Propria  (proper  layer)  or  substance  of  the  cornea. 
It  is  formed  of  some  sixty  strata  of  connective  tissue  bun- 
dles. These  cannot  be  stripped  ofif  in  layers,  but  are  made 
out  by  the  microscope  from  the  fact  that  the  connective 
tissue  bundles  run  in  different  directions ;  that  is,  for  in- 
stance, in  one  strata  all  the  bundles  run  vertically  across 
the  cornea,  the  next  layer  may  run  horizontally,  while  the 


Fig.  54.    Ciliary  region,  magnified  1,000  times. 

third  strata  may  have  its  bundles  laying  at  45°  or  135°. 
However,  there  is  such  a  free  exchange  of  bundles  from 
one  layer  to  another,  in  which  they  become  lost,  that  the 
whole  sixty  are  practically  as  one ;  so  that  this  arrangement 
forms  a  very  firm,  unyielding  structure.  At  the  sclero- 
corneal  juncture  or  limbus,  U,  Fig.  54,  the  lamina  propria 
continues  backward,  forming  the  sclerotic  by  continuity 
of  tissue,  the  difference  being  simply  that  the  nature  of  the 


84  THE    ANATOMY    OF    THE    EYE. 

tissue  is  changed  at  the  Hmbiis,  for  in  the  cornea  it  has  no 
blood  vessels,  while  the  sclera  is  fairly  well  supplied  with 
blood  vessels.  In  the  cornea  the  tissue  is  quite  dense  and 
transparent,  and  in  the  sclerotic  it  is  more  loosely  arranged 
and  is  opaque.  In  the  cornea  it  is  highly  supplied  with 
sensory  nerves,  while  in  the  sclerotic  it  has  only  a  moderate 
nerve  supply,  and,  farther,  if  we  should  examine  the  cornea 
chemically,  we  would  'find  it  contained  chondron  ( found  in 
cartilage),  while  the  sclerotic  would  chow  no  chondron,  but 
in  its  place  we  would  find  gelatin.  It  will  then  be  seen 
that  while  one  blends  into  the  other,  yet  the  two  tissues  are 
very  much  different. 

The  cornea  is  said  to  fit  into  the  sclera  as  a  watch  glass 
fits  into  the  bezel  of  a  watch.  This  impression  is  given 
from  the  fact  that  the  corneal  tissue  passes  farther  back- 
ward at  its  center,  while  the  sclera  runs  forward  farther 
at  its  outer  and  inner  surfaces.  When  we  view  this  with 
the  microscope  in  stained  sections,  it  appears  as  shown  at 
U,  Fig.  54.  This  is  from  the  fact  that  the  cornea  being 
more  dense  than  ihc  sclerotic,  it  retains  less  of  the  stain  in 
its  preparation,  so  that  we  can  make  out  the  limits  of  the 
two  tissues  fairly  well  in  this  way. 

Lying  within  the  lamina  propria  is  a  network  of  open- 
ings or  lymph  channels,  the  lacunae,  I,  Fig.  53  (small  lakes) 
and  the  minute  canals  (caniliculae)  which  run  out  in  all  di- 
rections from  the  lacunae  and  join  the  caniliculae  from 
surrounding  lacunae.  Lying  in  these  lacunae,  I,  Fig.  53, 
yet  not  entirely  filling  them,  are  found  the  fixed  or  corneal 
corpuscles.  These  cells  in  turn  have  very  minute  proto- 
plasmic processes  which  run  through  the  caniliculae  and 
join  or  anastomose  with  the  processes  from  the  cells  in  the 
neighboring  lacunae.  These  processes  do  not  entirely  fill 
the  caniliculae  in  which  they  lie;  thus  it  will  be  seen  that 
we  have  a  network  of  lymph  channels  through  which  the 
l>lood  plasma,  or  nutrient  lymph  can  have  free  passage  to 
all  parts  of  the  cornea  to  supply  it  with  nutrition.     This 


THE    ANATOMY    OF    THE    EYE^.  85 

lymph  is  given  off  from  the  capillary  loops,  forming  :i  circle 
around  the  margin  of  the  cornea,  which  will  be  described 
later. 

Passing  inward,  the  next  layer  is  Decimet's  Membrane, 
or  the  internal  homogenous  lamina,  D,  Fig.  53.  This  is  a 
very  thin,  highly  transparent  layer  and  has  a  tendency  to 
curl  up  when  stripped  off  of  the  cornea.  When  viewed 
with  the  microscope,  it  is  impossible  to  make  out  any  ground 
work,  it  seeming  to  be  wholly  made  up  of  a  hornlike  mem- 
brane, but  as  with  Bowman's  membrane,  if  treated  properly, 
to  remove  the  gelatinous  substance  which  forms  the  matrix 
or  joins  the  component  tissues  together,  it  will  be  found  to 
be  formed  of  connective  tissue.  Many  functions  have  been 
ascribed  to  this  membrane,  but  the  chief  one  is  its  great 
resistance  to  disease,  such  as  corneal  ulcers,  etc.  Some 
writers  claim  this  membrane  breaks  up  into  connective  tis- 
sue bundles,  bridges  across  the  filtration  angle  and  forms 
the  pectinate  (comb)  ligament,  K,  Fig.  54. 

This  is  composed  of  hundreds  of  connective  tissue  bun- 
dles which  run  from  the  periphery  of  the  cornea  to  the  base 
of  the  iris,  K,  Fig.  54,  and  A,  Fig.  53.  This  angle  formed 
by  the  iris  and  cornea,  V,  Fig.  54,  is  known  as  the  filtration 
angle  from  the  fact  that  the  aqueous  fluid  passes  out  of  the 
anterior  chamber  between  the  bundles  of  tissue,  forming  the 
pectinate  ligament,  to  the  spaces  of  Fontana  (fountain 
spaces),  which  comprises  the  openings  in  the  pectinate  liga- 
ment. The  posterior,  or  fifth  layer,  is  known  as  the 
endothelial  layer,  E,  Fig.  53.  This  is  formed  of  a  single 
layer  of  cubical  (square)  endothelial  cells,  which  are  placed 
like  paving  blocks  and  are  similar  to  cells  which  are  found 
in  other  parts  of  the  body,  lining  closed  cavities,  or  cavities 
which  have  no  opening  communicating  with  the  external 
world.  These  cells  have  the  faculty  to  withstand  the  dis- 
solving qualities  of  the  aqueous  fluid,  or  nutrient  lymph, 
which  fills  the  anterior  chamber. 


86  THE    ANATOMY    OF    THE    EYE. 

Some  anatomists  divide  the  cornea  into  three  portions; 
the  conjunctival  portion,  consisting  of  the  anterior  stratified 
epitheHum,  and  Bowman's  Membrane;  the  scleral  portion, 
consisting  of  the  lamina  propria ;  and  the  choroidal  portion, 
consisting  of  Decimet's  Membrane  and  the  endothelial  layer. 
This  is  from  the  fact  that  these  layers  are  supposed  to  be 
derived  from  these  structures. 

The  sclerotic  (tough)  coat,  I,  Fig.  54,  forms  the  posterior 
five-sixths  of  the  outer  coat  of  the  eyeball,  except  a  small 
opening  at  the  posterior  pole,  where  the  optic  nerve  pierces 
it.  This  opening  is  known  as  the  choroidal  fissure.  See 
Figs.  23,  56  and  57.  The  sclerotic,  as  before  stated,  is 
continuous  with  the  cornea  by  continuity  of  tissue. 
Just  outside  of  the  sclerotic  is  the  space  of  Tenon, 
X,  Fig.  54,  and  N,  Fig.  35.  This  is  a  space  be- 
tween the  capsule  of  Tenon  and  the  sclerotic.  The  cap- 
sule of  Tenon  forms  a  fibrous  socket  for  the  eyeball,  and 
this  space  of  Tenon  is  a  lymph  space  and  is  crossed  by  many 
connective  tissue  bundles  passing  from  the  capsule  to  the 
sclerotic.  These  are  known  as  Trabeculae.  Internal  to 
the  sclerotic,  between  it  and  the  choroid,  is  another  lymph 
space  known  as  the  suprachoroidal  space,  W,  Fig.  54. 
This  is  also  crossed  by  an  abundance  of  trabeculae  passing 
from  the  sclerotic  to  the  choroidal  coat.  In  fact,  the 
trabeculae  are  so  numerous  that  it  is  almost  impossible  to 
separate  the  two  structures.  The  sclerotic,  as  its  name  im- 
plies, is  very  tough  and  opaque.  The  innermost  portion 
contains  quite  a  little  pigment.  It  has  four  layers,  from 
without  inward ;  they  are  the  endothelial  layer  lining  the 
space  of  Tenon,  which  is  a  single  layer  of  pavement  cells. 
Next  comes  the  lamina  propria  (proper  layer)  ;  the  next 
layer  is  called  the  lamina  fusca  (Brown  layer).  The  lamina 
propria  and  the  lamina  fusca  are  not  sharply  defined  by  any 
line  of  demarcation,  but,  as  before  stated,  the  innermost 
strata  contains  some  pigment.  It  is  therefore  brown  in 
color,  the  pigment  not  being  sufficient  to  cause  it  to  appear 


THE    ANATOMY    OF    THE    EYE. 


«7 


black.  This  pigment  is  deposited  in  branched  cells.  The 
innermost,  or  fourth  layer,  is  the  internal  endothelial  layer, 
lining  the  supra  choroidal  space,  and  is  of  the  usual  pave- 
ment variety.  The  lamina  propria  and  lamina  fusca  are 
formed  of  tough  fibrous  tissue,  the  strands  of  which  run 


Fig.  55.    Magnified  2,500  times. 


in  all  directions  with  a  general  anterior  posterior  arrange- 
ment. Lying  in  the  substance  of  the  sclerotic  are  found 
lacunae,  the  same  as  in  the  cornea  which  contains  the  fixed 
or  scleral  corpuscles,  analogous  to  the  corneal  corpuscles. 
In  fact,  the  sclerotic  is  very  similar  to  the  cornea  in  the 
arrangement  of  the  connective  tissue,  except  that  it  is  not 
learly  so  compact. 


88  THE    ANATOMY    OF    THE    EYE. 

The  sclera,  as  before  stated,  is  well  supplied  with  blood 
vessels.  These  run  forward  and  end  in  capillary  loops  at 
the  limbus  and  form  a  complete  circle  extending  clear 
around  the  periphery  of  the  cornea.  They,  with  the  circle 
of  capillaries  formed  by  the  conjunctival  vessels,  F,  Fig.  54, 
and  I,  Fig.  52,  near  the  outer  surface,  give  off  the  nutrient 
lymph  which  flows  through  the  lacunae  (small  lakes)  and 
caniliculi  (minute  canals)  and  permeates  the  cornea.  This 
lymph  furnishes  the  nutrition  for  the  cornea.  To  give  the 
reader  an  idea  of  what  is  meant  by  capillary  loops,  we  have 
taken  a  microphotograph  of  an  injected  section  from  the 
sole  of  the  foot.  Fig.  55,  A.  This  is  the  Rete  Mukosum 
(capillary  layer,  or  malpighian  layer),. of  the  skin,  showing 
the  fine  capillaries  running  up  and  forming  loops  and  pass- 
ing back  as  venous  capillaries.  B  shows  some  elevations, 
which  form  little  ridges,  which  can  be  seen  on  the  ball  of 
the  thumb  so  readily.  C  shows  the  branch  of  an  artery, 
which  breaks  up  into  these  small  capillaries. 

At  J,  Fig.  54,  is  seen  the  canal  of  Schlemm.  This  is  a 
canal  lying  near  the  inner  surface  of  the  sclerotic,  just  at  the 
limbus.  It  is  circular  in  course,  running  clear  around  the 
margin  of  the  cornea.  It  may  be  single,  or  may  be  com- 
posed of  several  small  canals.  They  branch  from  and  re- 
turn to  the  main  opening,  so  that  it  forms  one  continuous 
sinus,  it  is  lined  with  endothelial  cells,  and  is  drained  by 
the  anterior  ciliary  veins.  The  aqueous  humor  passes  from 
the  spaces  of  fontana  to  the  canal  of  Schlemm  and  eventu- 
ally is  carried  back  into  the  circulation  by  the  anterior 
ciliary  veins. 

As  before  stated  the  sclerotic  forms  the  posterior  five- 
sixths  of  the  outer  coat  of  the  eyeball.  To  it  are  attached 
the  six  recti  (straight)  muscles.  See  Figs.  35,  36  and  37. 
It  is  pierced  by  the  anterior  ciliary  arteries  and  veins  at 
these  points  of  attachment.  (See  D,  Fig.  39.) 
These  pass  through  about  eight  to  ten  millimeters  back 
of  the  limbus;    then  just  back  of  the  equator  it  is  pierced 


THE    ANATOMY    OF    THE    EYE. 


89 


by  the  vena  vorticosa  (whorl  veins)  four  to  six  in  number. 
(See  H,  Fig.  39,  and  B.  and  C,  Fig.  40)  Then  posteri- 
orly it  is  pierced  by  the  cihary  arteries  and  nerves,  there 
being  twelve  to  twenty  of  each.  These  pass  through  just 
outside  of  the  optic  nerve  A,  Fig.  39,  and  at  A,  Fig.  56,  is 
seen  one  of  these  vessels  passing  through  this  structure. 
When  the  sclerotic  reaches  the  optic  nerve,  it  divides  into 


Fig.  56. 


three  portions.  The  innermost,  B,  Fig.  56  and  57,  breaks  up 
into  individual  bundles.  These  pass  across  the  choroidal  fis- 
sure and  form  the  lamina  cribrosa  (sieve  layer),  C,  Figs.  56 
and  57.  These  bundles  pass  across  in  all  directions  and  rein- 
force the  eyeball  at  this  otherwise  weak  point,  leaving  meshes 
or  openings  through  which  the  optic  nerve  fibers  pass  out  of 
the  eyeball.  It  is  also  pierced  by  the  arteria  centralis  retinae 
(central  artery  of  the  retina)  L,  Fig.  57.  The  opening 
through  the  lamina  cribrosa,  through  which  the  arteria  cen- 
tralis retinae  and  vein  pass,  is  known  as  the  porus  opticus. 


90 


THE    ANATOMY    OF    THE    EYE. 


The  middle  portion  passes  to  and  blends  with  the  pia  mater 
of  the  optic  nerve  D,  Figs.  56  and  57.  The  outer  portion 
passes  into  the  sheath  of  the  optic  nerve,  F,  Fig.  56  and  57. 
At  E  is  shown  the  intervaginal  space  of  the  optic  nerve, 
which  is  continuous  with  the  sub-dural  space  of  the  brain  at 
the  optic  foramen  and  contains  cerebro  spinal  fluid. 


Fig.  57. 

H,  Figs.  56  and  57,  shows  the  choroid,  and  J,  Figs.  56 
and  57,  shows  the  retina  detached  from  the  choroid.  K 
shows  the  physiological  cup ;  M  shows  the  optic  nerve,  and 
in  Fig.  57  the  nerve  bundles  are  extremely  well  shown  with 
the  myelin  sheaths  surrounding  them.  These  sheaths  end 
normally  just  behind  the  lamina  cribrosa,  C. 

The  choroid  is  continuous  from  the  optic  nerve  to  the 
free  margin  of  .the  iris,  or  to  the  pupillary  opening.  It  lies 
inside  of  the  schlerotic  and  is  the  second  grand  tunic  or  coat 
of  the  eye.     From  the  ora  serratta  of  the  retina  (saw  tooth 


THE    ANATOMY    OK    THE    EYE. 


91 


mouth  of  the  retina)  to  the  choroidal  fissure  it  Hes  in  touch 
with  the  sclerotic,  only  being  separated  from  it  by  the  supra 
choroidal  space  and  intimately  attached  to  the  sclerotic  by 
the  interchange  of  trabeculae  passing  across  the  supra  chor- 
oidal space  from  one  to  the  other.  It  is  a  pigmented  and 
highly  vascular  tissue,  as  its  name  implies,  and  supplies  the 
greater  part  of  the  nutrition  and  secretions  of  the  eyeball. 


Fig.  58.    Showing  Section  of  Choroid. 


It  is  composed  of  five  layers  of  fibrous  tissue  with  branched 
pigment  cells  in  the  meshes  between  the  connective  tissue 
fibers.  First  from  without  inward  we  have  the  endothelial 
layer  lining  the  supra  choroidal  space,  A,  Fig.  58.  Below 
that  is  the  lamina  supra  choroidea  (upper  layer  of  the 
choroid),  B,  Fig.  58,  also  called  the  lamina  fusca  of  the 
choroid,  on  account  of  its  pigmentation  and  brown  color. 


92 


THE    ANATOMY    OF    THE    EYE. 


The  next  layer  is  the  layer  of  large  blood  vessels,  C,  Fig. 
58.  The  next  layer  is  known  as  the  layer  of  small  blood  ves- 
sels, D,  Fig.  58.    The  layer  of  large  and  small  blood  vessels 


Fig.  59. 


Showing  Ora  Seratta,  Ciliary  Processes  and  Bodies  and  the 
Iris  from  Posterior  Aspect. 


are  composed  of  the  posterior  ciliary  arteries  as  they  pass  for- 
ward in  the  structure,  giving  off  branches  all  along  their 
course ;  also  the  veins  which  go  to  form  the  vena  vorticosa 
lie  in  these  two  layers.    The  next  layer  is  called  the  chorio 


THE    ANATOMY    OF    THE    EYE. 


93 


capillario  (capillary  layer  of  the  choroid),  E,  Fig.  58. 
These  capillaries  are  separated  from  the  retina  by 
the  bacillary  layer  (basement  layer),  also  called 
Bracks'  Membrane  or  lamina  vitrea,  F,  Fig.  58.  The 
capillary    layer    of    the    choroid    furnishes    much    of    the 


y 


!.  1        '.' 


Fig. 


Showing  Same  as  Fig.  59  in  Cross  Section. 


nutrition  to  the  outer  layers  of  the  retina,  it  reaching  them 
by  osmosis  (passing  through)  Brucks'  Membrane.  This 
layer  is  free  from  pigment  and  is  rich  in  cement  substance, 
so  much  so  that  it  is  a  homogeneous  membrane  or  layer. 

The  choroid  is  highly  pigmented  to  prevent  the  light  pene- 
trating the  wall  of  the  eyeball,  thus  making  an  absolutely 
dark  chamber  of  it.  The  choroid  is  extremely  susceptible  to 
disease  on  account  of  its  extreme  vascularity. 


94 


THE    ANATOMY    OF    THE    EYE. 


As  before  stated  the  choroid  is  continuous  from  the  head 
of  the  optic  nerve  forward  to  the  free  margin  of  the  iris. 
However,  it  is  divided  into  the  choroid  ciHary  process, 
ciHary  bodies  and  iris.  The  choroid,  I,  Fig.  60,  extends 
from  the  optic  nerve  to  the  ora  serratta  or  anterior  margin 
of  the  retina,  A,  Figs.  59  and  60.  It  then  becomes  somewhat 
ridged  on  its  inner  surface,  B,  Figs.  59  and  60.  These  ridges 
have  an  anterior  posterior  direction,  and  these  ridges,  about 
seventy   in   number,    are   known   as   the   ciHary   processes. 


Fig.  61.    The  CapiUaries  of  the  Ciliary  Processes. 


These  end  in  blunt  endings  which  project  into  the  cavity  of 
the  eyeball  towards  the  lens,  H,  Fig.  60,  and  are  known  as 
the  ciliary  bodies,  C,  Figs,  59  and  60.  From  the  outer  angle 
of  the  bases  of  the  ciliary  bodies,  J,  Fig.  60,  the  choroid  or 
uvea  leaves  the  outer  wall  of  the  eyeball  and  takes  a  trans- 
verse direction.  This  transverse  portion  is  called  the  iris 
(rainbow),  D,  Figs.  59  and  60.  At  the  center  of  the 
(Doll  so  called  from  the  diminutive  image  of  oneself  as 
seen  in  the  pupillary  area  when  looking  into  anyone's  eye), 
transverse  portion  there  is  an  opening  known  as  the  pupil, 


THE    ANATOMY    OF    THE    EYE. 


95 


G,  Figs.  59  and  60.  The  free  margin  of  the  iris,  F,  Figs.  59 
and  60,  Hes  free  and  rests  on  the  anterior  surface  of  the  lens, 
H,  Fig.  60.  The  short,  posterior  ciliary  arteries  run  forward 
through  the  choroid  in  the  layer  of  large  blood  vessels,  C, 
Fig.  58,  and  B,  Fig.  39,  being  bunched  in  straight  vessels  in 


Fig.  62.    The  Blood  Vessels  of  the  Iris. 


the  ciliary  processes  and  end  in  capillary  tufts,  Fig.  61,  in 
the  ciliary  bodies,  turning  back  as  venous  capillaries,  A, 
Fig.  40.  From  the  capillaries  are  given  out  the  fluids  of 
the  blood  which  passes  into  the  canal  of  Petit,  N,  Fig.  60, 


96  THE    ANATOMY    OF    THE    EYE. 

and  the  posterior  chamber,  M,  Fig.  60.  This  fluid  is  known 
as  the  aqueous  humor. 

The  two  long  posterior  ciliary  arteries  run  forward 
in  the  choroid  in  the  layer  of  large  blood  ves- 
sels, C,  Fig.  58,  and  C,  Fig.  39.  They  join  the 
anterior  ciliary  arteries,  D,  Fig.  39,  and  form  one 
arterial  trunk,  which  lies  right  at  the  base  of  the  iris, 
A,  Fig.  62,  K,  Fig.  60,  and  E,  Fig.  39.  This  arterial  trunk 
formed  by  the  anastomosis  (joining)  of  the  long  posterior 
and  anterior  ciliary  arteries,  is  known  as  the  circulus  major 
(larger  circle)  of  the  iris,  A,  Fig.  62.  From  the  circulus 
major  is  given  off  branches  which  run  radially  inward  to- 
ward the  free  margin  of  the  iris,  B,  Fig.  62,  These  radially 
coursing  arteries  in  the  iris  may  be  likened  to  the  spokes  in 
a  wheel.  When  they  come  near  to  the  free  margin  of  the  iris, 
they  anastomose  (join)  and  form  another  circle  known  as 
the  circulus  minor  (smaller  circle)  of  the  iris,  C,  Fig.  62. 
From  the  circulus  minor  is  given  off  capillaries  which  run 
inward  toward  the  free  margin  of  the  iris.  They  double  back 
as  venous  capillaries,  D,  Fig.  62.  The  drainage  from  the 
iris  is  by  the  anterior  ciliary  veins  which  leave  the  eyeball  at 
the  attachments  of  the  extrinsic  muscles,  while  the  drain- 
age from  ciliary  bodies  and  the  rest  of  the  uveal  tract  is 
drained  by  the  vena  vortacosa  (vortex  veins).    See  Fig.  40, 

Anterior  to  the  ora  serratta  the  outermost  or  pigment  lay- 
er of  the  retina  continues  forward  in  two  layers  of  colum- 
nar epithelial  pigmented  cells  and  lines  the  inner  wall  of  the 
eyeball  over  the  pars  ciliaris,  retina,  ciliary  processes,  and 
bodies,  O,  Fig.  60,  also  continuing  over  the  posterior  sur- 
face of  the  iris,  clear  to  the  free  margin  at  F,  Fig.  60.  This 
is  known  as  the  retinal  portion  of  these  structures  and  the 
amount  of  pigment  contained  in  this  layer  over  the  posterior 
of  the  iris  largely  determines  the  color  of  the  eye,  for  if 


THE    ANATOMY    OF    THE    EYE.  97 

there  is  no  pigment  in  this  layer,  or  the  stroma  of  the  iris, 
we  would  have  the  pink  or  albino  eye.  If  there  is  a  small 
amount  of  pigment  in  the  retinal  portion,  then  we  would 
have  a  light  blue  eye ;  a  little  more  pigment  and  it  will 
produce  the  dark  blue  eye,  and  so  on  as  more  pigment  is 
deposited,  the  eye  is  gray,  brown  or  black.  However,  in  the 
brown  and  black  eyes  there  is  much  pigmentation  of  the 
stroma  of  the  iris. 

Fig.  63  shows  a  cross  section  of  an  iris  in  which  the 
retinal  portion,  A,  is  well  pigmented,  while  the  stroma,  B, 
has  but  a  small  amount  of  pigment.  This  would  have  a 
tendency  to  produce  a  light  grayish  color  when  the  iris  is 
viewed  from  the  front.  Fig.  64  shows  a  cross  section  of  an 
iris,  which  is  highly  pigmented,  both  in  the  retinal  layer,  A, 
as  well  as  the  stroma,  B.  This  would  produce  a  dark 
brown  or  black  iris  if  viewed  from  the  front. 

The  iris  is  a  very  delicate  structure  formed  of  a  very  thin 
network  of  connective  tissue,  with  a  large  amount  of  cells 
filling  in  the  spaces  between  the  connective  tissue  fibers. 
In  dark  eyes  these  cells  become  more  or  less  pigmented,  C, 
Fig.  64.  The  iris  contains  two  muscles,  the  Sphincter 
(binder)  Pupillae,  A,  Fig.  65,  and  the  Dilator  (enlarger) 
Pupillae,  B.  It  has  four  layers  from  within  outward ;  they 
are  the  pigment,  or  retinal  layer,  F,  muscular,  B,  the  stroma 
proper  in  which  lie  the  blood  vessels,  E,  and  the  endothelial 
or  corneal  layer,  D.  The  front  of  the  iris  has  deep  depres- 
sions or  crypts ;  these  run  radially,  or  from  the  base  toward 
the  free  margin.  These  depressions,  or  crypts,  lie  between 
the  blood  vessels,  see  Fig.  62,  and  in  medium  or  light  col- 
ored eyes  this  causes  the  stellate  (star  like)  or  radially 
spoke-like  appearance  of  the  anterior  surface  of  the  iris  as 
seen  in  those  eyes.  Fig,  66  represents  a  quadrant  of  the 
front  surface  of  an  iris;  A,  the  pigment  layer  at  the  free 
margin ;  B,  the  circulus  minor  and  capillaries ;  C,  the  circu- 


98  THE    ANATOMY    OF    THE    EYE. 


Fig.  63.    Cross  section  from  iris  of  a  light  colored  eye. 


Fig.  64.    A  cross    section  of  Iris  from  dark  Eye. 


THE    ANATOMY    OF    THE    EYE. 


99 


lus  major;  D,  a  trabeculae  or  ridge  in  which  runs  a  blood 
vessel;  E,  a  depression  or  crypt;  and  F,  the  pectinate  liga- 
ment. The  spoke  or  stellate  appearance  is  caused  by  the 
vessels  being  so  near  the  surface  that  the  reflection  is  great- 
er over  them  than  from  the  spaces  between  them.  How- 
ever, in  very  dark  eyes,  the  pigment  is  so  densely  deposited 


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■ 

.1 

Fig.  65.    Showing  cross  section  of  Iris,  its  muscles  and  layers. 


that  it. hides  the  blood  vessels;  therefore,  in  dark  eyes  this 
spokelike  appearance  is  absent. 

Lying  just  behind  the  iris  is  found  the  crystalline  lens 
(pea  or  lentil),  and  as  the  name  implies,  it  is  a  transparent 
body  shown  at  A,  Fig.  6y.  This  lies  in  the  Fossae  Patilaris 
(dish  like   depression)    in    the    anterior    surface    of    the 


lOO 


THE    ANATOMY    OF    THE    EYE, 


vitreous  body  and  is  held  in  place  by  the  suspensory  liga- 
ment, B.  The  lens  has  two  portions,  however,  not  divisible 
or  sharply  outlined;  the  central  or  nuclear  portion  and  the 
outer  or  cortical  portion.  The  central  or  nuclear  portion 
is  more  dense  than  the  cortical  portion.     The  nuclear  por- 


Fig.  66.    A  quadrant  of  the  front  surface  of  the  Iris. 


tion  is  composed  of  the  elongated  or  spindle  cells  which 
first  fill  the  lens  vesicle  by  the  elongation  of  the  cells  com- 
posing the  posterior  wall  of  the  lens  vesicle;  see  Fig.  8. 
These  cells,  as  before  stated,  fill  the  whole  cavity  as  shown 
in  Fig.  68,  D.  The  nuclei  of  these  cells  are  pushed  forward, 
as  shown  at  K.  After  these  first  formed  spindle  cells  have 
filled  the  lens  vesicle,  then  at  the  transitional  (transform- 
ing) zone,  the  original  cells  of  the  lens  vesicle  continue  to 
elongate  and  grow  around  the  ends  of  the  cells  which  have 
formed  the  nucleus  of  the  lens  as  shown  at  J,  and  in  this 
section    the    cells,    which    will    form    the   cortical   portion, 


THE    ANATOMY     OF    THE    EYE. 


lOl 


are  just  beginning  to  grow  and  elongate.  These  cells 
then  form  the  outer  or  cortical  portion  of  the  lens  and  the 
ends  of  the  fibers  butt  together,  as  shown  at  A,  Fig.  69. 
These  fibers,  or  spindle  cells,  have  a  diamond  shape  and 


Fig.  67.    Cross  section  of  the  liuman  eye. 


these  again  are  formed  in  layers  bound  together  by  trans- 
parent cement  substance.  These  layers  are  then  laid  one 
on  another,  as  the  layers  of  an  onion  are  found,  and  these 
layers  in  turn  are  bound  together  by  the  cement  substance. 
Over  the  anterior  surface  of  the  crystalline  lens  is  formed 
a  single  layer  of  columnar  cells,  which  are  the  cells  com- 
posing the  original  lens  vesicle  wall.  This  layer  extends 
back  to  the  equator  of  the  lens  and  then  they  become  trans- 


I02  THE    ANATOMY    OF    THE    EYE. 

formed  into  the  spindle  cells,  which  compose  the  lens  sub- 
stance. The  area  of  transformation  is  known  as  the  transi- 
tional Zone;  see  G,  Fig.  60.  Surrounding  the  whole  lens 
is  a  thin  transparent  membrane  known  as  the  capsule  of 
the  lens,  C,  Fig.  67,  and  to  this  capsule  is  attached  the  sus- 
pensory ligament,  the  anterior  fibers  just  in  front  of  the 
equator  and  the  posterior  fibers  just  behind  the  equator. 


Fig.  68.    Human  embryo  eye,  2  months.     Magnified  1,080  times. 

The  suspensory  ligament  of  the  lens  or  Zonule  or  Zinn. 
C  and  D,  Figs.  70  to  74,  is  imbedded  in  the  outer  layer  of 
the  hyaloid  membrane.  This  membrane  divides  into  two 
layers  at  the  ora  seratta  of  the  retina  (saw  tooth  mouth), 
F,  Figs.  70  to  74.  The  inner  layer  continues  over  the  front 
of  the  vitreous  body,  while  the  outer  layer  in  which  the 
fibers  of  the  suspensory  ligament,  I,  Figs.  70  to  74,  are  im- 
bedded, is  firmly  bound  down  to  the  inner  surface  of  the 


THE    ANATOMY    OP^    THE    EYE.  IO3 

pars  ciliaris  retina,  ciliary  processes  and  bodies  G  and  H, 
Figs.  70  to  74.  From  the  ciliary  processes,  H,  the  fibers 
and  membrane  leave  the  outer  wall  of  the  eyeball  and  turn 
transversely  across  toward  the  equator  of  the  lens,  P. 
The  outer  layer  of  the  hyaloid  membrane,  I,  Figs.  70  to 
73,  becomes  very  thin  and  fluid  passes  through  it  very 
readily.     It  passes  across  with  the  fibers  of  the  suspensory 


Fig.  69.    Human  embryo  eye,  5  months.    Magnified  7,000  times. 

ligament,  which  are  attached  in  front  of  the  equator  of  the 
lens,  and  the  triangular  space  bounded  by  it  in  front  and 
the  hyaloid  membrane  behind  with  its  base  at  the  equator 
of  the  lens.  The  apex  at  the  ciliary  bodies  is  called  the 
canal  of  Petit,  E,  Figs.  70  to  73. 

The  fibers  of  the  suspensory  ligament,  C  and  D,  Figs.  70 
to  74,  arise  from  the  retina  at  the  ora  seratta,  F,  and  are 
continuous  clear  to  their  attachments  to  the  lens,  A  and  B. 
These  fibers  are  believed  to  be  specialized  elongated  fibers 


104 


THE    ANATOMY    OF    THE    EYE. 


Fig.  70.    Showing  the  suspensory  Hgament.' 


Fig.  71.    Showing  ora  seratta  and  ciliary  processes. 


THE    ANATOMY    OF    THE    EYE.  I05 

of  Mueller,  which  are  of  a  very  elastic  nature.  These  fibers 
become  attached  to  the  lens  capsule  during  the  develop- 
ment of  the  eye  and  as  the  eye  enlarges  become  elongated. 
When  they  leave  the  ciliary  bodies  they  divide  and  a  part 


Fig  72.    Enlarged  view  of  ciliary  muscle. 

of  them,  C,  Figs.  70  and  73,  pass  to  their  attachment  to 
the  lens  capsule  in  front  of  the  equator  and  others,  D,  Figs. 
70  to  74,  pass  to  their  attachment  to  the  lens  capsule  back 
of  the  equator,  while  a  few  pass  across  in  the  canal  of 
Petit,  E,  Figs.  70  to  73.    These  are  attached  to  the  lens  at 


io6 


THE     ANATOMY    OF    THE     EYE. 


its  equator.  By  glancing  at  Fig.  70  and  noting  the  attach- 
ment of  the  suspensory  ligament,  C  and  D,  it  will  readily 
be  understood  that  tension  on  the  suspensory  ligament, 
C  and  D,  of  the  lens,  P,  Fig.  70,  would  have  a  tendency  to 
flatten  it  in  its  anterior  posterior  diameter  and  enlarge  its 


■ 

S^^^ 

n 

^^^^H 

^ 

^     T 

I 

Fig.  73.    Showing  ciliary  processes  and  bodies. 


transverse    diameter,    thus    increasing   and    decreasing    its 
convexity,  as  this  tension  was  exerted  or  relaxed. 

Lying  between  the  ciliary  processes,  bodies  and  choroid, 
G,  H  and  I,  Figs.  70  to  73,  and  the  sclerotic,  K,  Figs,  ^^2 
and  y^,  is  found  the  ciliary  muscle,  L  and  M,  Figs.  70  to  73 
(hair-like  muscle),  composed  of  plain  muscular  fibers.  It 
is  composed  of  two  portions,  the  longitudinal  or  the  outer 
portion,  L,  Figs.  70  to  yi,  and  the  circular  portion,  M, 
Figs.  70  to  73.  The  fibers  of  the  first  or  outer  portion,  L, 
run  anterior  posterior,  arising  at  the  limbus    (seam),    N, 


THE    ANATOMY     OB     THE    EYE. 


107 


Figs.  70  and  "j^i^  a  portion  of  them  in  front  of  and  a  portion 
posterior  to  the  canal  of  Schlemm,  O,  Figs.  70  and  73. 
The  circular  portion,  M,  has  the  same  origin,  but  takes  a 
circular  course  and  lies  just  ouiside  of  the  ciliary  bodies,  H, 
Figs.  70  to  y2).  The  longitudinal  fibers  are  attached  to 
the  outer  surface  of  the  choroid,  J,  Figs.  71  to  y2i-     The) 


Fig.  74.    Suspensory  ligaments  and  lens  drawn  in. 


spread  out  fan  shaped,  some  being  attached  as  far  forward 
as  the  posterior  ends  of  the  ciliary  bodies,  H,  Figs.  70  to 
74.  Others  extend  backward  and  arc  attached  as  far  back- 
ward as  the  ora  scratta,  F,  Figs.  70  to  74;  thus  it  is  seen 
they  have  a  very  extensive  attachment  to  the  choroid. 
The  function  of  the  ciliary  muscle  is  to  put  the  choroid,  J, 


Io8  THE    ANATOMY    OF    THE    EYE. 

on  the  stretch.  This  is  possible  owing  to  the  supra 
choroidal  space,  Figs.  70  to  73,  separating  the  choroid 
and  sclerotic,  and  the  circular  fibers,  M,  press  the  ciliary 
bodies,  H,  nearer  to  the  equator  of  the  lens,  B,  Fig.  74. 
As  the  suspensory  ligament,  C  and  D,  is  bound  down  to  the 
choroid  ciliary  processes  and  bodies,  and  bridges  across  the 
space  between  the  ciliary  bodies,  H,  and  the  lens,  P,  the 
action  of  the  muscle  when  it  contracts  is  to  slacken  the 


Fig.  75.    Vitreous  darkened  to  show  hyaloid  canal. 

suspensory  ligament,  allowing  the  lens  P  to  become  more 
convex  by  virtue  of  its  elasticity  or  resiliency.  However, 
the  main  strain  in  accommodation  seems  to  fall  upon  the 
circular  portion  M  in  Figs.  70  to  y^,  from  the  fact  that  in 
myopic  eyes  where  the  far  point  is  within  thirteen  inches 
of  the  eye,  where  accommodation  is  never  necessary,  but 
few  if  any  of  these  circular  fibers  are  found  upon  exam- 
ining the  ciliary  muscle  after  death,  whereas  in  the  hyper- 
metropic eyes  where  accommodation  is  necessary  for  all 


THE    ANATOMY    OF    THE    EYE.  IO9 

vision,  these  fibers  will  be  found  to  be  very  plentiful.  In 
fact  they  have  been  known  to  make  up  as  much  as  seventy- 
five  per  cent  of  the  bulk  of  the  muscle.  The  ciliary  muscle 
receives  its  nerve  supply  from  the  posterior  ciliary  nerves, 
which  arise  from  the  lenticular  or  ciliary  ganglion  (to  knit 
or  weave),  which  receives  its  motor  roots  from  the  third 
cranial  or  motor  oculi  nerve.  See  Figs.  42,  43  and  44. 
The  terminal  portions  of  the  posterior  ciliary  nerves  break 
up  into  small  anastomosing  branches  and  form  the  ciliary 
plexus,  which  lies  in  the  ciliary  muscle. 

The  vitreous  body,  B,  Fig.  75,  composes  the  greater  por- 
tion of  the  eyeball,  filling  all  the  cavity  posterior  to  the  lens. 
It  is  composed  of  shapeless  transparent  cells,  very  loosely 
arranged,  so  that  it  resembles  a  sponge  and  is  filled  with 
fluid  resembling  the  aqueous  humor,  and  is  about  of  the 
density  as  the  white  of  an  egg,  running  through  the 
vitreous  body.  Antero  posteriorly  from  the  posterior  of 
the  lens  to  the  head  of  the  optic  nerve  is  found  a  lymph 
canal.  A,  Fig.  75,  which  was  the  space  occupied  by  the 
hyaloid  artery,  which  is  present  during  the  development  of 
the  lens  during  foetal  life.  See  B,  Fig.  16.  This  canal  is 
known  as  the  hyaloid  canal  or  the  canal  of  Stilling.  The 
lens  is  imbedded  in  the  anterior  surface  of  the  vitreous 
body,  lying  in  a  depression  called  the  Fossae  Patellaris 
(saucer-like  depression),  C,  Fig.  75.  The  whole  body  is 
surrounded  by  the  hyaloid  membrane  (glass-like  mem- 
brane), which  is  transparent  and  homogenous  (structure- 
less), D,  Fig.  75.  This  membrane  divides  at  the  ora  ser- 
atta,  F,  Figs.  70  to  74,  the  inner  layer  covering  the  an- 
terior of  the  vitreous  and  lining  the  fossae  patellaris,  C, 
Fig.  75,  while  the  outer  layer  is  intimately  attached  to  the 
ciliary  processes  and  bodies  and  leaves  the  ciliary  bodies 
and  extends  to  the  lens.  In  this  outer  layer  is  imbedded 
the  fibers  of  the  suspensory  ligament,  I,  Figs.  70  to  73. 
Posterior  to  the  ora  seratta  the  hyaloid  membrane  is  very 
intimately  attached  to  the  retina,  F,  Fig.  75.    This  attach- 


no  THE    ANATOMY    OF    THE    KYE. 

ment  is  so  firm  that  when  the  vitreous  body  is  disturbed  the 
nine  innermost  layers  of  the  retina  are  usually  detached. 

The  retina  (net)  lines  the  inner  wall  of  the  eye  ball, 
it  extends,  properly  speaking,  from  the  head  of  the  optic 
nerve,  M.  Fig.  'j^),  to  the  Ora  Serratta  (Saw  Tooth  Mouth) 
X ;  however,  it  is  continuous  clear  to  the  free  margin  of 


Fig.  76.     Cross  section  of  the  human  eye,  showing  its  construction. 

the  Iris  E,  by  means  of  a  double  layer  of  pigmented 
epithelial  cells  which  cover  the  inner  surface  of  the  pars 
ciliaris  retina  (the  part  between  the  ciliary  bodies  and  the 
retina),  G,  Fig.  ^^,  ciliary  processes  G,  Fig.  74,  and  ciliary 
bodies  H.  Figs.  73  and  74,  as  well  as  the  inner  or  pos- 
terior surface  of  the  iris,  E.  Fig.  76.  This  anterior  or  pig- 
mented  portion   is   called   the   Uvea    (Grape   Skin)  ;   it   is 


THE    ANATOMV"    OF    THE    EYE. 


II  I 


formed  by  the  continuation  forward  of  the  outer  or  pig- 
ment layer  of  the  retina  and  the  anterior  portion  of  the 
secondary  optic  vesicle  which  does  not  take  part  in  the 
formation  of  the  nine  innermost  layers  of  the  retina  or 
more  properly  speaking,  the  receiving  and  transmitting 
portion  of  this  structure. 


Fig.  77. 


The  retina  is  a  very  thin,  delicate  structure,  being  one- 
half  millimeter  thick  at  its  thickest  portion  near  the  optic 
nerve,  and  gradually  becoming  thinner  toward  the  ora 
serratta,  where  it  is  but  one-tenth  millimeter  thick.  It  is 
firmly  attached  to  the  choroid  at  the  ora  serratta,  and  is 
firmly  bound  down  at  the  head  of  the  optic  nerve  by  virtue 
of  the  optic  fibers  passing  from  it  through  the  choroidal 
fissure  (the  opening  of  the  choroid),  L.  Fig.  "jd.  There  is 
a  less  secure  attachment  at  the  macula  lutea  (yellow  spot), 


112  THE    ANATOMY    OF    THE    EYE. 

J.  Fig.  78.  In  all  other  portions  of  the  retina  the  nine  in- 
nermost layers  are  very  loosely  attached  to  the  outer  or 
pigment  layer;  this  attachment  is  accomplished  simply  by 
the  interlacing  of  the  rods  and  cones  with  the  processes 
which  project  inward  from  the  cells  forming  the  pigment 
or  outer  layer.  It  is  held  in  place  mainly  by  the  inter- 
ocular  pressure. 


Fig.  78.    Cross  section  of  the  Eye,  showing  its  construction. 

The  retina  receives  its  blood  supply  from  the  arteria 
centralis  retina  (central  artery  of  the  retina)  which  reaches 
it  through  the  choroidal  fissure  after  having  traversed  the 
optic  nerve  for  some  ten  millimeters  back  of  the  eye  ball, 
L.  Fig.  yy.  This  artery  is  an  end  artery,  or  in  other  words, 
it  is  not  joined  by  any  other  set  of  arteries,  but  it  sends  its 
branches  to  all  parts  of  the  retina,  A.  Fig.  78,  terminat- 


THE    ANATOMY    OF    THE    EYE. 


113 


ing  ill  arterial  capillaries  and  turning  back  as  venous  capil- 
laries ;  these  keep  joining  and  rejoining  and  form  the  vena 
centralis  retina  (the  central  vein  of  the  retina),  which 
leaves  the  eye  ball  through  the  choroidal  fissure  by  the 
side  of  the  entrance  of  the  artery.     See  darker  vessels  in 


%- 


Fig.  79. 

Fig.  78.  By  staining  cross  sections  of  the  retina  it  is 
shown  to  be  divisable  into  ten  layers.  Seven  of  these  are 
nervous  tissue,  two  of  neuroglia  or  nervous  connective  tis- 
sue and  one  of  pigmented  epithelium.  The  nine  innermost 
layers  are  transparent  and  are  bound  together  by  the  fibers 


114  '^"^    ANATOMY    OF    THE    EYE. 

of  Mueller,  which  is  the  nervous  connective  tissue  of  the 
retina.  The  outermost  or  pigmented  layer  is  more  inti- 
mately attached  to  the  choroid  than  it  is  to  the  other  layers. 

The  layers  from  within  outward  are:  First,  the  inner 
limiting  membrane,  A.  Fig.  79.  Second,  the  layer  of  nerve 
fibers,  B.  Third,  the  layer  of  ganglionic  cells  or  gang- 
lionic (knot  like)  layers,  C.  Fourth,  the  mner  molecular 
or  plexiform  layer,  D.  Fifth,  the  inner  nuclear  or  granular 
layer,  E.  Sixth,  the  outer  molecular  or  plexiform  layer,  F. 
Seventh,  the  outer  nuclear  or  granular  layer,  G.  Eighth, 
the  outer  limiting  layer,  H.  Ninth,  the  layer  of  rods  and 
cones,  I.  Tenth,  the  pigment  layer,  J.  K.  shows  the  hya- 
loid membrane  which  lies  just  inside  of  the  retina  and  L 
shows  the  choroid  ^which  is  the  structure  just  outside  of  the 
retina.  In  this  section  the  choroid  is  somewhat  torn  and 
separated. 

The  pigment  layer,  as  before  stated,  is  composed  of  a 
smgle  layer  of  columnar  epithelial  cells  which  are  long 
hexagonal  cells  separated  from  each  other  by  a  well  de- 
fined, clear,  cement  substance.  They  have  long  proto- 
plasmic processes  which  project  inward  and  interlace  with 
the  rods  and  cones.  In  these  cells  are  deposited  pigment 
granules  which  remain  in  the  base  or  outer  portions  of  the 
cells  when  the  eye  is  closed  or  in  darkness.  See  G,  Fig. 
80.  F  is  the  lamina  vitrea  or  Bruck's  membrane  of  the 
choroid.  However,  when  the  retina  is  exposed  to  the  light 
these  pigment  granules  flow  into  the  processes  which  lie 
amongst  the  rods  and  cones  (C,  Fig.  81),  thus  protecting 
these  delicate  structures  from  destruction  by  too  intense 
light  as  well  as  forming  a  screen  right  amongst  the  rods 
and  cones,  to  receive  the  image  which  is  formed  by  the 
refracting  surfaces  of  the  eye.  See  Fig.  81.  A  is  the 
choroid,  B  the  bases  of  the  pigment  cells  and  C  the  pro- 
cesses lying  amongst  the  rods  and  cones. 

The  layer  of  rods  and  cones  (I  Fig.  79),  especially  the 
cones,  are  the  real  sensory  cells  of  the  retina,  as  it  is  their 


THE    ANATOMY    OF    THE    EYE. 


"5 


function  to  produce  the  impulse  which  is  transmitted  to 
the  brain  and  there  produces  the  sense  of  sight.  Each  rod 
and  each  cone  is  at  the  end  of  a  process  which  comes  from 
a  cell  in  the  outer  nuclear  layer  (G,  Fig.  79).  These  pass 
through  openings  in  the  outer  limiting  membrane  (H,  Fig. 


Fig.  80.    Showing  Section  of  Choroid. 

79).  The  cones,  as  their  name  implies,  are  of  a  conical 
shape,  shorter  than  the  rods ;  they  have  a  large  oval  inner 
portion  with  a  finer  tapering  point  extending  outward 
into,  and  interlacing  with  the  processes  extending  inward 
from  the  pigment  layer.  The  oval  or  enlarged  inner  por- 
tion is  striated  longitudinally,  while  the  outer  or  tapering 
portion  is  formed  apparently  of  discs.  The  rods  are  long 
cylindrical  cells  striated  longitudinally,  and  are  divided  into 
two  segments  at  about  their  middle.  Their  function  is 
not  clearly  established.     There  are  estimated  to  be  about 


ii6 


THE    ANATOMY    OF    THE    EYE. 


three  niilHon  cones  in  the  human  retina,  and  the  rods  ex- 
ceed this  many  times.  The  cones  predominate  in  the  ma- 
cula or  most  acute  area  of  sight,  while  the  rods  predom- 
inate in  all  other  portions  of  the  retina,  thus  proving  the 
cones  to  be  the  real  sensory  elements. 

The  next  layer  from  without  inward  is  the  outer  limit- 
ing layer.  (EP,  Fig.  79.)  This  is  formed  by  the  overlap- 
ping of  the  flattened  ends  or  feet  of  the  outer  extremities 


Fig.  81. 


of  the  fibers  of  Mueller  or  nervous  connective  tissue,  which 
will  be  explained  later;  this  layer  is  punctured  by  millions 
of  openings  through  which  pass  the  processes  on  the  distal 
ends  of  which  the  rods  and  cones  are  found.  The  next 
layer  inside  of  the  outer  limiting  membrane  is  the  outer 
nuclear  or  granular  layer.  (G,  Fig.  79.)  It  is  almost 
wholly  composed  of  bipolar  cells;  that  is,  they  have  two 
processes,  one  runs  outward  through  the  outer  limit- 
ing membrane  and  ends  in  a  rod  or  cone,  whilst  the  other 
runs  inward  and  ends  in  a  brush  like  end  or  tuft  in  the 
outer  molecular  layer.  The  single  layer  of  cells  seen  just 
inside  of  the  inner  limiting  membrane  in  Fig.  79,  are  sup- 


THE    ANATOMY    OF    THE    EYE.  II7 

posed  to  be  the  cells  connected  with  the  cones  whilst  the 
cells  connected  with  the  rods  lie  in  the  middle  and  inner 
portion  of  this  layer.  There  are  several  varieties  of  nerve 
cells  found  in  this  layer,  the  functions  of  which  are  un- 
determined, and  will  be  omitted  in  this  description.  The 
next  innermost  layer  is  the  outer  molecular  or  plexiform 
layer.  (F  Fig.  79.)  This  layer  is  composed  of  the  end  ar- 
borisations of  the  bipolar  cells  in  the  outer  nuclear  layer, 
which  run  inward,  and  the  distal  end  tufts  on  the  processes 
from  the  bipolar  cells  in  the  inner  nuclear  layer,  which 
run  outward,  as  well  as  some  other  nerve  cells  which  have 
processes  which  extend  to  a  greater  or  less  extent  in  this 
layer.  They  are  known  as  amacrine  (long  fiber)  cells; 
their  function  is  undetermined,  but  they  seem  to  be  asso- 
ciation elements  to  join  different  portions  of  the  same 
layer. 

The  next  innermost  layer  is  the  inner  nuclear  or  granular 
layer,  E,  Fig.  79.  This  layer  is  mainly  formed  of  bipolar 
cells;  they  send  one  process  outward  into  the  outer  mole- 
cular layer  which  ends  in  a  brush-like  end  or  tuft  interlac- 
ing with  the  tufts  on  the  inner  ends  of  the  inner  processes 
from  the  bipolars  of  the  outer  nuclear  layer  and  send 
another  process  inward  into  the  inner  molecular  layer 
which  ends  in  an  end  tuft  or  arborisation.  There  are  other 
nerve  cells  in  this  layer  also,  the  function  of  which  has  not 
been  determined.  The  next  innermost  layer  is  the  inner 
molecular  or  plexiform  layer,  D,  Fig.  79.  This,  like  the 
outer  plexiform  layer,  is  almost  wholly  composed  of  the 
end  tufts  of  the  processes  from  the  bipolar  cells ;  however 
these  come  from  the  bipolars  in  the  inner  nuclear  layer 
which  run  inward  and  the  processes  which  run  outward 
from  the  ganglionic  cells  in  the  ganglionic  layer  and,  as 
explained  about  the  other  cells  found  in  the  outer  molecular 
layer,  those  found  in  the  inner  molecular  layer  have  not 
been  thoroughly  studied  and  their  functions  ascertained 
farther  than  that  they  associate  different  areas  of  the  same 


Il8  THE    ANATOMY    OF    THE    EVE. 

layer.  The  next  innermost  layer  is  the  ganglionic  (knot- 
like) cell  layer,  C.  These  cells  might  well  be  called  relay 
cells,  for  they  are  very  large ;  they  send  from  two  to  three 
processes  outward  into  the  inner  molecular  layer  from 
each  cell,  which  form  tufts  and  interlace  with  the  tufts  on 
the  inner  ends  of  the  processes  from  the  bipolar  cells  in  the 
inner  nuclear  layer.  It  is  from  these  ganglionic  cells  that 
the  axis  cylinder  processes  grow  which  form  the  next  inner- 
most layer,  which  is  called  the  nerve  fiber  layer,  B.  These 
axis  cylinder  processes  are  continuous  from  the  ganglion 
cells  of  the  retina  into  the  nerve  fiber  layer.  They  pass  out 
of  the  eyeball  through  the  choroidal  fissure  and  form  the 
optic  nerve,  which  will  be  described  later,  and  are  con- 
tinuous from  the  ganglion  cells  in  the  retina  to  the  nuclei 
at  the  base  of  the  brain.  The  next  innermost  layer  is  the 
inner  limiting  membrane,  A.  It  is  formed  by  the  expand- 
ed or  foot-like  inner  ends  of  the  fibers  of  Mueller.  The 
fibers  of  Mueller  are  the  sustentacular  (sustaining  or  bind- 
ing) tissue  of  the  retina  and  are  the  same  as  the  neuroglia 
cells  found  in  the  brain  and  spinal  cord.  They  are  long, 
branching,  connective  tissue  cells  which  extend  from  the 
inner  to  the  outer  limiting  membranes  and  the  overlapping 
of  their  expanded,  or  foot-like,  ends  form  both  the  inner 
and  outer  limiting  membranes.  Their  function  is  to  bind 
the  nine  innermost  layers  of  the  retina  together.  The 
retina  becomes  quite  thin  at  the  macula  and  the  cells  which 
otherwise  would  occupy  the  space  are  piled  up  around  it. 
The  processes  from  these  displaced  cells,  as  well  as  the 
fibers  of  Mueller,  run  obliquely  outward  and  toward  its 
center. 

The  optic  nerve,  M,  Figs,  "j^  and  'JJ,  leaves  the  eyeball 
at  the  choroidal  fissure  (opening  through  the  choroid)  and 
is  made  up  of  the  axis  cylinder  processes,  which  arise  from 
the  ganglionic  layer  of  the  retina  C,  Fig.  79,  and  lie  be- 
tween this  layer  and  the  inner  limiting  membrane,  A,  form- 
ing the  nerve  fiber  layer  of  the  retina  B.     These  nerve 


THE    ANATOMY    OF    THE    EYE. 


II 


9 


fibers,  or  axis  cylinder  processes,  pass  through  the  openings 
in  the  lamina  cribrosa  (sieve  layer),  C,  Fig.  77,  just  back 
of  the  choroidal  fissure.  The  fibers  are  bare,  or,  in  other 
words,  devoid  of  the  myeline  (marrow)  sheaths  or  white 
substance  of  Sw^an,  until  after  they  pass  through  the  lamina 
cribrosa    (sieve  layer).     This  covering  is  then  added  and 


Fig. 


Cross  section  of  optic  nerve  showing  neuroglia    stained  dark 
and  nerve  fibers  light. 


this  addition  adds  greatly  to  the  bulk  or  size  of  the  nerve 
at  the  choroidal  fissure  and  at  points  posterior  to  the  lamina 
cribrosa.  All  the  fibers  which  arise  from  the  ganglionic 
cells  in  the  retina  transmit  visual  impulses  toward  the 
brain.  However,  in  the  optic  nerve  are  found  many  fibers 
which  grow  from  the  brain  to  the  retina.    These  are  sensory 


i20 


tHk    ANAtOMY    Ot'    illE    eyB;> 


ifibers  of  association  and  carry  sensory  impulses  which  cause 
the  closure  of  the  pupil  when  the  retina  is  exposed  to 
bright  light,  as  well  as  causing  the  dilation  of  the  pupil 
when  the  eye  is  in  darkness  and  govern  co-ordinate  move- 
ments of  the  two  eyes. 


Fig.  83.      Cross  section  of  optic  nerve  showing  nerve   fibers  stained 
dark  and  the  neuroglia  stained  light. 


The  arteria  centralis  retina  (central  artery  of  the  retina), 
L,  Fig.  ^"j,  B,  Fig.  78,  and  H,  Fig.  82,  and  the  vena  cen- 
tralis retinae  (central  vein  of  the  retina),  I,  Fig.  82,  and 
dark  vessels  in  Fig.  78,  enter  and  leave  the  eyeball  with 
the  optic  nerve,  after  entering  its  substance  some  ten  or 
twelve  millimeters  back  of  the  eyeball. 


THE    ANATOMY    OF    THE    EYE. 


121 


The  optic  nerve  is  surrounded  by  three  coverings ;  the 
outermost  being  the  optic  nerve  sheath,  A,  Figs.  82  and 
83,  and  F,  Fig.  yy.  This  covering  is  formed  by  the  con- 
tinuation backward  around  the  nerve  of  the  outermost 
portion  of  the  sclerotic,  Y,  Fig.  jd,  and  F,  Fig.  yj.  This 
sheath  is  continuous  backward  to  the  optic  foramen  (open- 


Fig.  84.    Showing  cross  section  of  the  head  of  a  bird, 

mg),  where  it  is  continuous  with  the  dura  mater  (hard  or 
firm  mother)  of  the  brain.  The  optic  nerve  sheath  is 
quite  firm  and  is  composed  of  connective  tissue  bundles. 
Beneath  the  optic  nerve  sheath  is  found  a  space  surround- 
ing the  nerve  which  is  known  as  the  intervaginal  space, 
E,  Fig.  jy,  and  B,  Figs.  82  and  83.  This  space  is  con- 
tinuous through  the  optic  foramen  with  the  sub-dural  and 
sub-arachnoidal  spaces  of  the  brain,  and  this  intervaginal 
space  is  filled  with  the  cerebro  spinal  fluid. 


122  THE    ANATOMY    OF    THE    EYE. 

Lying  within  the  intervaginal  space  is  found  the  arach- 
nodial  sheath  (spider  web  sheath),  G.  Fig.  82.  This  is  a 
very  thin,  web-like  membrane,  joined  quite  intimately  to 
the  outer  and  inner  sheaths  of  the  optic  nerve,  by  trabeculse 
(beams),  which  cross  the  intervaginal  space. 

The  innermost  covering  or  sheath  is  known  as  the  pia 
mater  (thin  mother)  or  pial  sheath,  D,  Fig.  "jj,  and  C, 
Figs.  82  and  83.  This  is  formed  of  glial  tissue  (nervous 
connective  tissue)  and  from  it  is  given  off  the  septa  or 
trabeculae  (beams)  which  surrounds  the  bundles  of  nerve 
fibers  and  forms  the  frame  work  of  the  optic  nerve  and 
holds  it  together,  E,  Figs.  82  and  83,  and  darker  longi- 
tudinal striations  in  M,  Fig.  J'j,  The  pial  sheath  and 
trabeculse  is  highly  supplied  with  minute  arteries  and  veins 
which  furnish  it  with  nutrition. 

The  optic  nerve  is  composed  of  about  eight  hundred 
bundles  of  medulated  (covered  with  myelin)  nerve  fibers, 
D,  Figs.  82  and  83,  and  light  longitudinal  striations  in  M, 
Fig.  77,  each  bundle  being  composed  of  from  six  to  seven 
hundred  axis  cylinder  processes  or  nerve  fibers,  each  of 
which  are  insulated  or  covered  by  the  myelin  (marrow) 
sheaths. 

The  optic  nerves,  B,  Fig.  84,  leave  the  eyeballs.  A,  Fig. 
84,  just  internal  to  the  posterior  poles  of  the  eyeballs,  and 
run  obliquely  backward  and  inward  through  the  orbit  and 
pass  into  the  cranial  cavity  through  the  optic  foramen, 
then  join  together  and  form  the  optic  commissure  (uniting 
band),  C,  Fig.  84.  In  the  commissure  a  part  of  the  nerve 
fibers  decussate  (cross  over)  and  pass  backward  in  the 
optic  tract  of  the  opposite  side,  while  a  portion  pass  into 
the  optic  tract  of  the  same  side. 

The  optic  tracts  extend  from  the  optic  commissure  to 
the  base  of  the  brain,  where  a  part  of  the  optic  fibers  enter 
the  external  and  internal  geniculate  (knee-like)  bodies, 
others,  the  optic  thalmus  (bed),  and  the  rest  go  to  the 
anterior  corpora  quadrigemina  (meaning  the  four  bodies). 


THE    ANATOMY    OP^    THE    EYE.  I23 

These  latter  fibers  are  supposed  to  be  the  sensory  associa- 
tion fibers,  which  communicate  with  the  different  centers 
of  the  brain  and  their  function  is  for  co-ordinate  move- 
ments of  the  two  eyes  as  well  as  reflex  movements  and 
sensibilities,  while  the  optic  fibers  which  enter  the  other 
basilar  (lower)  nuclei  (nut)  come  in  contact  with  the  proto- 
plasmic processes  of  the  ganglion  (enlarged  or  swollen) 
cells  in  these  bodies.  From  these  ganglion  cells  extend 
the  axis  cylinder  processes,  which  run  upward  and  back- 
ward through  the  optic  radiations  to  reach  the  centers  of 
sight  which  are  situated  along  the  calcarian  fissure  in  the 
cuniate  lobe  of  the  brain,  which  is  located  in  the  posterior 
or  occipital  region.  It  is  by  the  interpretation  of  the  im- 
pulses created  by  the  cones  in  the  retina  and  transmitted 
through  the  conducting  elements  in  the  retina,  optic  nerve, 
optic  commissure,  optic  tracts,  external  and  internal  genicu- 
late bodies,  optic  thalmus,  and  optic  radiations  to  these 
centers,  that  sight  is  accomplished  by  man. 


The  Physiology  of  Vision 


PREFACE. 

These  lectures,  with  the  exception  of  the  first  and  last, 
were  delivered  before  the  Chicago  Optical  Society.  As  the 
members  of  this  society  were  familiar  with  the  eye  as  a 
dioptric  mechanism,  the  subjects  of  refraction  and  the  errors 
of  refraction  were  treated  very  briefly,  the  time  being  de- 
voted chiefly  to  a  popular  exposition  of  the  sensation  of 
vision. 

While  the  aim  has  been  to  present  these  lectures  in  as  sim- 
ple a  manner  as  possible,  yet  it  is  the  author's  conviction 
that  popular  lectures  ought  not  to  depart  from  the  general 
course  of  scientific  methods ;  experimentation  and  observa- 
tion ought  to  precede  the  drawing  of  conclusions, and  knowl- 
edge should  be  obtained  at  first  hand  whenever  possible. 
For  this  reason  and  also  to  increase  the  interest  in  the  sub- 
ject, a  large  number  of  experiments  have  been  introduced. 
These  experiments  are  of  such  a  simple  character  that  the 
reader  will  find  little  difficulty  in  performing  them. 

If  these  lectures  and  experiments  shall  stimulate  the 
reader  to  a  greater  interest  in  the  study  of  the  human  body, 
the  author  shall  feel  that  the  aim  of  these  lectures  has  been 
accomplished. 

Chicago,  February  13,  1906.  W.  D.  Z. 


12' 


LECTURE  I. 

Spencer  defines  life  as  the  continuous  adjustment  of  in- 
ternal to  external  relations.  The  external  relations  of  a  plant 
or  animal  change  continually,  and  some  of  these  changes 
are  of  such  a  nature  that  unless  the  organism  brings  itself 
into  harmony  with  these  changes,  its  life  is  in  danger.  To 
enable  the  organism  to  adjust  internal  to  external  relations, 
it  must  be  informed  of  the  external  changes ;  this  is  accom- 
plished by  sense  organs,  such  as  the  ear,  the  eye,  etc.,  which 
are  capable  of  being  stimulated  or  afifected  by  the  changes 
in  the  environment.  The  so-called  special  sense  organs  are 
highly  developed  organs;  they  are  so  highly  modified  that 
they  are  generally  stimulated  by  only  one  particular  kind  of 
stimulus.  Thus  the  ear  is  usually  stimulated  only  by  the 
sound  waves  of  the  air ;  the  eye,  by  light. 

Light  is  the  vibration  of  ether  (see  Lecture  II).  When 
light  waves  fall  upon  a  bright  surface  they  are  reflected  in 
such  a  manner  that  the  angle  of  incidence  is  equal  to  the 
angle  of  reflection  (Fig.  i).  This  law  is  true  whether  the 
reflecting  surface  is  a  plane  surface,  like  an  ordinary  mirror, 
or  a  curved  surface  like  a  concave  or  convex  mirror.  In 
tracing  the  reflected  ray  from  a  concave  or  convex  surface 
it  is  necessary  to  remember  that  the  normal  (perpendicular) 
to  a  spherical  surface  is  the  radius  extending  from  the  cen- 
ter to  the  point  where  the  incident  beam  strikes  the  surface. 
This  is  illustrated  in  Fig.  2  where  x  is  the  center  of  the  re- 
flecting surface  ab.  Cd  is  the  incident  beam  striking  the 
surface  at  d.  We  may  now  draw  the  normal  xd  and  the 
angle  between  the  lines  cd  and  dx  is  the  angle  of  incidence. 
Lay  ofif  an  equal  angle  on  the  other  side  of  dx,  this  is  the 
angle  of  reflection,  and  de  is  the  reflected  ray. 

129 


130 


THE    PHYSIOLOGY    OF    VISION. 


Light  does  not  travel  with  the  same  velocity  in  all  media ; 
in  air  it  travels  at  the  rate  of  186,000  miles  per  second;  in 
a  denser  medium,  as  in  glass,  its  velocity  is  less.  The  ratio 
of  the  velocity  of  light  in  air  to  that  in  crown  glass  is  as  3 


Fig.l 


is  to  2;  hence  light  travels  1.5  times  as  fast  in  air  as  in 
crown  glass.  For  this  reason  the  optical  density  of  crown 
glass  is  said  to  be  1.5. 

When  light  passes  from  an  optically  rarer  into  an  optical- 
ly denser  medium,  as  for  example,  from  air  into  water  or 
into  glass,  it  is  refracted,  or  bent,  toward  the  normal  (per- 


Fig.2 

pendicular),  and  when  it  passes  from  a  denser  into  a  rarer 
medium,  it  is  bent  away  from  the  normal.  This  is  illustrated 
in  Fig.  3.  The  bending  of  the  ray  takes  place  at  the  surface 
separating  the  two  media  of  different  optical  density ;  hence 
this  surface  is  called  a  refracting  surface.     A  ray  of  light 


THE    PHiSIOI.OGY    OF    VISION. 


»3i 


that  is  normal  to  the  refracting  surface  undergoes  no  re- 
fraction. 

The  same  law  holds  true  when  light  falls  upon  a  convex 
lens.     Suppose  the  light  comes  from  an  infinite  distance ;  in 


Fig.  3 


this  case  the  rays  of  light  may  he  regarded  as  parallel.  In 
Fig.  4  let  xy  he  a  convex  lens  and  let  the  centers  of  the  sur- 
faces be  located  at  o  and  o\  The  ray  ah  is  normal,  to  both 
surfaces  of  the  lens  (it  passes  through  the  two  centers)  and 


Fig.  4 


is  therefore  not  refracted.  The  ray  cd  is  not  normal  to  the 
surface  and  is  therefore  bent  toward  the  normal  (do")  and 
describes  the  course  de.  The  ray  de  in  passing  out  of  the 
lens  meets  the  posterior  refracting  surface  and  is  bent  away 
from  the  normal  (eo)  and  pursues  the  course  eh.  In  the 
same  manner  the  ray  pq  is  refracted  so  that  it  describes  the 
course  rs.  It  will  be  noticed  that  the  three  rays  ab,  cd  and 
pq  after  passing  through  the  lens  cross  each  other  at  the 


132  THE    PHYSIOLOGY    OF    VISION. 

point  f.  This  point,  where  parallel  rays  come  to  a  focus, 
is  called  the  principal  focus,  and  the  distance  between  the 
lens  and  the  principal  focus  is  called  the  focal  length  of  the 
lens. 

If  the  source  of  light  is  situated  at  a  point  nearer  than 
infinity,  the  rays  cannot  be  considered  parallel  and  the 
focus  of  this  luminous  point  lies  further  away  from  the 
lens  than  the  principal  focus.  In  Fig  5  let  f  be  the  principal 
focus.  If  the  source  of  light  is  situated  at  1,  the  focus  is  at 
b.  The  converse  is  also  true,  if  b  is  the  luminous  point,  the 
focus  is  at  1.  Hence  the  points  1  and  b  are  called  conjugate 
foci.  If  the  distance  from  the  luminous  point  1  to  the  lens 
is  twice  the  focal  length  of  the  lens,  then  the  distance  be- 
tween b,  the  focus,  and  the  lens  is  also  equal  to  twice  the 


Fig.  5 


focal  length.  As  the  point  1  approaches  the  lens,  the  dis- 
tance between  b  and  the  lens  increases ;  as  1  recedes  from 
the  lens,  b  approaches  it. 

The  distance  between  the  lens  and  the  focus  of  any  lum- 
inous points  can  readily  be  calculated     from    the  formula: 

I  I  I  I  I  I 

1         b  ~  f        b        f         1 

in  which  f  is  the  focal  length,  1  the  distance  between  the 
luminous  point  and  the  lens,  and  b  the  distance  between 
the  focus  and  the  lens.  Suppose  the  focal  distance  of  a  lens 
is  six  centimeters  (or  inches),  and  suppose  a  luminous  point 
is  situated  ten  centimeters   (or  inches)   from  the  lens;  the 


THE    PHYSIOLOGY    OF    VISION. 


33 


distance  between  the  focus  of  this  huiiinous  point  and  the 

lens  is  therefore : 

I         I         I  I 


10 


15 


that  is,  b,  the  distance  between  lens  and  focus,  is  fifteen 
centimeters  (or  inches). 

From  this  it  is  obvious  that  (a)  if  the  luminous  point  is 
at  infinity,  the  focus  is  at  the  principal  focus;  (b)  if  the 
luminous  point  lies  nearer  to  the  lens  than  infinity,  but  fur- 
ther than  the  focal  distance  of  the  lens,  its  focus  lies  some- 
where between  the  principal  focus  and  infinity;  (c)  if  the 
luminous  point  is  situated  at  a  distance  equal  to  the  focal 
length  of  the  lens,  the  focus  lies  at  infinity,  i.  e.,  the  re- 


Fig.  6 

fracted  rays  are  parallel;  (d)  if  the  luminous  point  lies 
nearer  to  the  lens  than  the  focal  distance,  the  focus  lies  be- 
yond infinity,  that  is,  it  does  not  exist  and  the  refracted  rays 
are  divergent.  These  four  cases  are  illustrated  in  Fig.  6 
in  which  F  is  the  principal  focus. 

How  can  we  now  determine  the  location  of  the  focus  of 
any  luminous  point  ?  In  Fig.  7  let  f  and  f  be  the  principal 
foci  of  the  lens  and  let  1  be  the  luminous  point.  Draw  a  ray 
of  light  (la)  from  1  parallel  with  the  optical  axis.  This  ray 
after  refraction  passes  through  the  principal  focus  f.  Again, 


134 


THE    PHYSIOLOGY    OF    VISION. 


take  a  ray,  Ic,  from  1,  passing  through  the  principal  focus  f ; 
after  refraction  this  is  parallel  with  the  optical  axis.  These 
two  refracted  rays  cross  each  other  at  b  and  this  is  the  po- 
sition of  the  focus  of  the  luminous  point  1.  It  will  be  no- 
ticed that  we  can  draw  a  ray  from  1  through  the  point  o 
which  on  prolonging  meets  the  other  two  rays  at  b ;  that  is, 
this  ray  is  not  refracted.  The  point  o  is  called  the  nodal  point 


C^li^UfaA 


OyfCA^ 


Fig.  7 

and  may  be  defined  as  a  point  in  the  lens  of  such  a  nature 
that  a  ray  of  light  going  towards  it  is  not  refracted  in  pass- 
ing through  the  lens.  If  the  position  of  the  image  of  an  ob- 
ject is  desired,  we  proceed  in  the  same  manner  as  above. 
From  Fig.  8  it  will  be  seen  that  the  image  of  a  convex  lens 
is  a  real  and  inverted  image. 


Fig.  8 


All  convex  lenses  do  not  have  the  same  focal  distance; 
that  is,  they  do  not  have  the  same  refractive  power.  The 
greater  the  optical  density  of  a  lens,  the  greater  the  refrac- 
tive power.  The  density  of  a  medium  is  called  its  index  of 
refraction.  Again  two  lenses  of  the  same  material  and 
therefore  of  the  same  index  of  refraction,  do  not  necessarily 


THE    PHYSIOLOGY    OF    VISION.  1 35 

have  the  same  power  of  refraction.  This  depends  upon  the 
deg-ree  of  curvature,  or,  in  better  words,  upon  the  radius 
of  curvature ;  the  shorter  the  radius,  the  greater  the  refrac- 
tive power.  Hence  the  law,  the  refractive  power  of  a  lens 
varies  directly  as  the  index  of  refraction  and  inversely  as 
the  radius  of  curvature. 

The  refractive  power  of  a  lens  may  be  stated  in  terms  of 
its  focal  length.  A  lens  which  has  the  principal  focus  lOO 
centimeters  from  the  lens  is  said  to  have  one  diopter  refrac- 
tive power.  If  a  lens  has  a  focal  length  of  fifty  centimeters 
its  refractive  power  is  equal  to  one  hundred  divided  by  fifty, 
or  two  diopters.  Again,  a  lens  of  four  diopters  has  a  focal 
length  of  one  hundred  divided  by  four,  or  twenty-five  centi- 
meters. 

We  are  now  ready  to  consider  the  eye  as  an  organ  of 
vision.  In  the  eye  we  have  two  groups  of  tissues ;  first,  a 
group  of  tissues  sensitive  to  light,  and,  secondly,  tissues 
by  which  the  rays  of  light  entering  the  eye  are  properly 
focussed  on  the  sensitive  tissues.  The  sensitive  tissue  is  the 
innermost  coat  of  the  eye-ball  and  is  called  the  retina  (See 
Fig.  14).  The  media  by  which  the  light  is  focucsed  are  the 
cornea,  the  aqueous  humor  filling  the  anterior  chamber,  the 
crystalline  lens,  and  the  vitreous  humor  (Fig.  14).  These 
media  are  transparent  and,  as  they  have  a  greater  density 
than  air,  the  light  in  passing  from  the  air  into  the  eye  is 
refracted.  How  much  the  light  is  refracted  depends,  as  we 
have  stated  above,  on  the  indices  of  refraction  of  the  optical 
media  and  on  the  radii  of  their  curvatures.  These  values 
are  stated  in  the  following  table. 

Index  of  refraction  of  cornea 1.33 

Index  of  refraction  of  aqueous  humor.  ...  1.33 

Index  of  refraction  of  lens 1.43 

Index  of  refraction  of  vitreous  humor.  ...  1.33 
(Index  of  refraction  of  air i.cxd) 


13^  THE    PHYSIOLOGY    OF    VISION. 

Radius  of  curvature : 

Of  anterior  surface  of  cornea R  mm. 

Of  anterior  surface  of  aqueous  humor.  .8  mm. 

Of  anterior  surface  of  lens lo  mm. 

Of  posterior  surface  of  lens 6  mm. 

i 

From  the  above  table  it  will  be  noticed  that  the  cornea 
and  the  aqueous  humor  have  the  same  index  of  refraction ; 
consequently  the  surface  between  these  two  media  is  not  a 
refracting  surface  and  may  be  neglected.  Between  the  air 
and  the  cornea,  between  the  aqueous  humor  and  the  lens, 
and  between  the  lens  and  the  vitreous  humor,  we  have  re- 
fracting surfaces  at  which  the  light  is  bent. 


Fig.  9 


The  general  course  of  the  rays  in  the  eye  is  diagramatic- 
ally  represented  in  Fig.  9.  The  rays  of  light  are  most  re- 
fracted at  the  cornea  because  of  the  great  difference  between 
the  index  of  refraction  of  air  (i.o)  and  that  of  the  cornea 
(1.33).  As  before  stated,  the  light  undergoes  no  refraction 
at  the  posterior  surface  of  the  cornea.  At  the  anterior  sur- 
face of  the  lens  the  light  is  refracted,  but  not  as  much  as  at 
the  cornea  for  the  difference  between  the  density  of  the 
aqueous  humor  (1.33)  and  that  of  the  lens  (143)  is  less 
than  the  difference  between  the  density  of  air   (i.o)   and 


THE    PHYSIOLOGY    OF     VISION.  I37 

that  of  the  aqueous  humor  (1.33).  Moreover,  the  radius 
of  curvature  of  the  anterior  surface  of  the  lens  (10  mm.) 
is  greater  than  that  of  the  cornea  (8  mm.).  At  the  pos- 
terior surface  of  the  lens  the  Hght  is  refracted  more  than  at 
its  anterior  surface,  because  the  radius  of  curvature  of  the 
posterior  surface  (6  mm.)  is  much  less  than  that  of  the  an- 
terior surface  (10  mm.). 

The  refractive  power  of  the  cornea  is  generally  stated  at 
about  43  and  that  of  the  lens  at  15  diopters,  making  the  to- 
tal refractive  power  of  the  eye  58  diopters. 

The  refracting  surfaces  and  the  retina  are  so  placed  that 
in  the  emmetropic  eye  (See  Lecture  II)  the  posterior  prin- 
cipal focus  falls  on  the  retina.     As  the  diameter  of  the  eye 


Fig.  10 

is  about  23  mm.,  we  may  say  that  the  posterior  principal 
focal  length  of  the  eye  (at  rest)  is  about  23  mm.  If  a 
point  of  light  is  placed  at  13  mm.  in  front  of  the  eye,  the 
rays  in  the  vitreous  humor  are  parallel ;  that  is,  the  anterior 
principal  focal  distance  is  13  mm. 

The  eye  also  has  a  nodal  point  lying  near  the  posterior 
surface  of  the  lens.  To  find  the  position  of  the  image  of  an 
object  we  make  use  of  this  nodal  point,  as  is  shown  in  Fig. 
10.  From  this  figure  it  will  be  noticed  that  the  image  on 
the  retina  is  inverted.  If  the  object  lies  to  the  left  of  the 
observer,  the  image  of  that  object  lies  on  the  right  half  of 
the  retina.  The  size  of  the  image  of  any  object  can  readily 
be  calculated  in  the  following  manner.  Let  the  object  be  a 
letter  on  this  page  which  is  about  1/20  or  .05  inch  tall,  and 
let  the  distance  between  the  letter  and  the  eye   (reckoned 


138  THE    PHYSIOLOGY    OF     VISION. 

from  o,  the  nodal  point,  in  Fig*.  10)  be  30  inches.    The  dis- 
tance  from  o  to  the  retina  is  about  0.6  inch.     Hence  the 

0.05  X0.6 

size  of  the  image  of  the  letter  on  the  retina  will  be 

I  30 

= inch. 

1000 


LECTURE    II. 

In  order  to  have  distinct  vision  four  requisites  are  neces- 
sary: first,  a  well  defined  image  of  the  object  must  be 
formed  on  the  retina ;  second,  a  change  in  the  retina  of  such 
a  nature  that  it  can  be  communicated  to  the  endings  of  the 
optic  nerve ;  third,  the  propagation  of  this  change  along  the 
optic  nerve  to  the  brain;  and,  lastly,  the  projection  into  space 
of  the  sensation  produced.  The  first  of  these  requisites  will 
be  considered  in  this  lecture. 


Fig.  11.    Diagram  showing  focus  of  far  (F)   and  near  (B)  object  when 
eye  is  at  rest. 


You  are  all  familiar  with  the  condition  of  the  emmetropic 
eye.  When  the  emmetropic  eye  is  at  rest,  parallel  rays  of 
light  come  to  a  focus  on  the  retina,  at  F,  in  Fig.  ii.  The 
rays  of  light  coming  from  a  far  off  object  are  regarded  as 
parallel,  consequently  the  emmetropic  eye,  when  at  rest,  sees 
distant  objects  distinctly.  The  rays  coming  from  a  near 
object  are  divergent,  and  divergent  rays  always  come  to  a 
focus  later  than  parallel  rays.  A  near  object,  as  the  point 
A,  Fig.  II,  therefore,  has  its  focus,  B,  back  of  the  retina  and 
is  consequently  not  seen  distinctly.  Yet  it  is  a  well  known 
fact  that  a  near  object  can  be  seen  distinctly  by  the  emme- 

139 


140  THE    PHYSIOLOGY    OF    VISION. 

tropic  eye.  In  order  that  the  near  object  shall  be  seen  dis- 
tinctly, a  change  must  be  induced  in  the  eye  of  such  a 
nature  that  its  focus,  B,  shall  fall  on  the  retina.  Bringing 
the  focus  of  a  near  object  upon  the  retina  is  called  accom- 
modation.    How  is  this  produced? 

It  is  interesting  to  note  the  various  ideas  that  have  been 
held  regarding  the  power  of  the  emmetropic  eye  to  see  near 
points.  We  may,  first  of  all,  notice  the  theory  of  Kepler 
(A.  D.  1600),  who  held  that  the  lens  in  viewing  near  objects 
moved  forward.  The  further  the  lens  moves  forward,  the 
nearer  the  focus,  B,  approaches  the  retina.  This  process 
actually  takes  place  in  some  animals  (snakes),  but  it  is 
not  possible  for  man  thus  to  see  near  objects,  for  even  if 
the  lens  should  move  as  close  as  possible  to  the  cornea,  still 
the  focus  of  a  near  object  would  lie  some  distance  behind 
the  retina.  About  the  same  time  or  a  little  later,  Scheiner 
put  forward  the  view  that  accommodation  was  brought 
about  by  the  constriction  of  the  pupil.  As  we  shall  see  in 
a  subsequent  lecture,  constriction  of  the  pupil  renders  the 
vision  of  near  objects  more  distinct,  but  it  is  impossible  by 
mere  constriction  of  the  pupil  to  cause  the  images  of  near 
..objects  to  be  as  distinct  as  they  ordinarily  are  during  near 
vision.  Besides  this,  vision  of  near  objects  is  possible  in 
case  of  absence  of  the  iris,  or  in  case  of  a  floating  iris,  or 
of  adherence  of  the  iris  to  the  cornea.  Arlt  held  that  ac- 
commodation was  brought  about  by  the  elongation  of  the 
eyeball.  Theoretically  the  focus  of  the  near  object  could 
thus  be  brought  on  to  the  retina,  as  the  focus  is  brought  on 
to  the  plate  in  a  camera.  That  such  a  mechanism  is  not 
used  in  the  eye  is  evident  from  the  fact  that  for  the  near 
point  the  eyeball  would  have  to  elongate  five  millimeters 
(1-5  inch).  The  rigidity  of  the  eyeball  is  sufficient  ground 
for  rejecting  this  view. 

It  has  also  been  held  that  vision  for  near  objects  was 
brought  about  by  decreasing  the  radius  of  curvature  of  the 
cornea ;  i.  e.,  increasing  its  convexity.     Now  decreasing  the 


THE    PHYSIOLOGY    OF    VISION. 


M 


radius  of  curvature  increases  the  refractive  power  and 
therefore  shortens  the  focal  distance,  i.  e.,  the  distance  be- 
tween lens  and  image.  Young,  in  the  early  part  of  1800, 
proved  that  the  eye  does  not  accommodate  in  this  manner, 
by  showing  that  accommodation  is  possible  when  the  eye  is 
submerged  in  water.  If  accommodation  for  a  near  point 
was  due  to  changes  in  the  cornea,  accommodation  could  not 
take  place  under  water  for  the  following  reasons. 


Fig.  12.    Diagram  showing  formation  of  the   Purkinje-Sanson  images 
and  their  relative  position  during  far  and  near  vision. 


In  order  that  a  surface  (like  the  anterior  surface  of  the 
cornea  or  lens)  shall  act  as  a  refracting  surface,  that  is, 
shall  have  the  power  to  bend  the  light,  it  must  be  the  bound- 
ary between  two  media  having  different  densities  (or 
indices  of  refraction).  The  indices  of  refraction  (density) 
of  air  and  cornea  are  i  and  1.37  respectively;  consequently 
the  light  is  bent  (refracted)  when  it  passes  from  air  into  the 
cornea.  As  water  has  an  index  of  1.33,  placing  the  eye  un- 
der water  destroys  the  refractive  power  of  the  cornea,  for  it 
now  no  longer  separates  two  media  of  different  densities, 


142  THE    PHYSIOLOGY    OF    VISION. 

and  hence  no  alteration  in  its  curvature  could  affect  the 
refraction.  Young  was  one  of  the  first  to  hold  that  accom- 
modation is  due  to  some  change  in  the  lens.  To  support  this 
idea,  he  stated  the  fact  that  accommodation  is  impossible  in 
the  absence  of  the  lens. 

It  was  finally  decided  by  means  of  the  Purkinje-Sanson 
images  that  the  changes  which  occur  in  the  eye  during  ac- 
commodation take  place  in  the  lens.  The  Purkinje-Sanson 
images  are  the  images  of  reflection  seen  in  the  eye. 

Experiment  i.*  Hold  a  candle  about  four  or  five  inches 
in  front  of  the  bridge  of  the  nose  of  another  person.  Look 
into  the  eye  from  the  temporal  side.  By  slightly  shifting 
the  position  of  the  candle  or  of  your  own  eye,  you  will  ob- 
serve three  images  of  the  candle.  Nearest  the  nose  you  will 
observe  a  large  and  very  bright  image  which  moves  up  as 
you  move  the  candle  upward.  On  the  temporal  side  you 
will  observe  a  very  small,  quite  bright  image  which  moves 
contrary  to  the  movement  of  the  candle.  In  between  these 
is  seen  a  large,  very  dim  and  ill  defined  image  which  moves 
in  the  same  direction  as  the  candle  is  moved.  This  image 
is  somewhat  difficult  to  find,  but  a  little  patience  will  reveal 
it.  Make  this  experiment  in  a  dark  place  and  have  the  ob- 
served eye  look  at  a  distant  object. 

These  three  images  are  formed  by  the  cornea  and  by  the 
anterior  and  posterior  surfaces  of  the  lens.  In  Fig.  12  the 
line  O  P  is  the  optical  axis  upon  which  lie  the  centers  of  the 
surfaces.  A  ray  of  light  leaves  the  candle.  A,  so  as  to  strike 
the  apex  of  the  cornea ;  this  is  reflected  into  the  observer's 
eye  at  B,  so  that  he  sees  an  image  of  the  candle.  This  image 
he  projects  into  the  interior  of  the  eye,  as  in  case 
of  a  mirror,  and  therefore  thinks  the  image  situated 
at  X.  Another  ray  of  light  leaves  the  candle  in 
such  a  manner  as  to  strike  the  anterior  surface  of  the  lens 
(drawn  in  full  line).     This  ray  is  also  reflected  into  the  ob- 


*The  experiments  described  in  this  series  of  lectures  either  were  per- 
formed during  the  lecture  or  are  of  such  a  simple  nature  that  they 
can  be  performed  at  home. 


THE    PHYSIOLOGY    OF    VISION.  I43 

server's  eye  and  a  second  image  is  supposed  to  exist  at  Y. 
Again,  a  third  ray  from  the  candle  goes  into  the  eye  and 
strikes  the  posterior  surface  of  the  lens  in  such  a  manner 
that  it  is  also  reflected  into  the  observer's  eye.  Consequent- 
ly three  images  are  seen,  X,  Y,  and  Z  (see  Fig.  13A)  ;  X 
being  produced  by  the  cornea,  Y  by  the  anterior  surface,  and 
Z  by  the  posterior  surface  of  the  lens.  As  X  and  Y  are 
reflected  from  convex  surfaces,  these  images  are  erect,  as 
can  be  seen  by  moving  the  candle  up  and  down;  Z  being 
formed  by  a  concave  surface  is  inverted,  moving  in  con- 

B 


Fig.  13.    Showing  relative  position  and  size  of  the  three  Purkinje-San 
son  images ;  A  during  far  and  B  during  near  vision. 

trary  direction  with  the  candle.  Of  these  three  images,  X 
is  very  bright  and  rather  large;  Y  is  the  largest  and  the 
dimmest ;  Z  is  the  smallest.  The  size  of  the  reflected  image 
varies  directly  as  the  radius  of  curvature  of  the  reflecting 
surface.  The  radius  of  curvature  of  the  cornea  is  eight 
millimeters,  that  of  the  anterior  surface  of  the  lens,  when 
the  eye  is  at  rest,  is  ten  millimeters  and  of  the  posterior 
surface  six  millimeters.  Consequently,  Z  produced  by  the 
posterior  surface  of  the  lens  is  the  smallest  image  and  Y 
produced  by  the  anterior  surface  of  the  lens  is  the  largest 
image.  If  now  (in  experiment  i)  the  observed  eye  accom- 
modates for  a  near  point,  it  will  be  noticed  that  the  image 
Y  produced  by  the  anterior  surface  of  the  lens  approaches 


144  THE    PHYSIOLOGY    OF    VISION. 

the  image  X  and  becomes  smaller.  Compare  Fig.  13A  and 
13B.  These  changes  in  this  image  can  only  be  explained 
by  assuming  that  during  accommodation  the  anterior  sur- 
face of  the  lens  decreases  its  radius  of  curvature  and  ap- 
proaches the  cornea.  In  Fig.  12  the  dotted  outline  of  the 
lens  represents  the  lens  in  a  condition  of  near  vision.  In  this 
case  we  must  take  a  new  ray,  the  dotted  ray,  from  the  can- 
dle to  the  anterior  surface  of  the  lens.  This  ray  is  also  re- 
fected into  the  observer's  eye  and  the  image  is  projected 
into  the  observed  eye,  so  that  it  is  now  located  at  Y',  that  is, 
nearer  to  X.  And  as  the  radius  of  curvature  has  also  de- 
creased, Y'  will  be  found  to  be  smaller  than  Y. 

As  we  stated  a  moment  ago,  decreasing  the  radius  of  curv- 
ature increases  the  refractive  power  and  shortens  the  focal 
distance;  consequently  the  focus  of  the  near  point,  which, 
in  the  resting  eye  fell  behind  the  retina,  B,  Fig.  11,  now  falls 
on  the  retina. 

The  next  subject  to  consider  is  how  this  change  is  pro- 
duced in  the  lens.  In  order  to  do  this  intelligently  the 
anatomy  of  the  eye  must  be  studied.  The  outer  coat  of  the 
eye  is  called  the  sclerotic.  Fig.  14.  Inside  of  this  is  located 
the  choroid,  which  contains  the  blood  vessels,  and  inside 
of  this  we  find  the  retina,  the  sensitive  layer  of  the  eye. 
Anteriorly  the  choroid  is  thickened,  so  as  to  form  the  iris, 
Fig.  14,  and  the  ciliary  processes.  These  processes  en- 
circle the  lens  and  from  them  extend  the  suspensory  liga- 
ments by  which  the  lens  is  held  in  place.  According 
to  the  Helmholtz  theory,  the  choroid  coat  of  the  eye  al- 
ways has  a  tendency  to  be  pushed  backward.  This  causes 
the  suspensory  ligaments  to  be  drawn  taut  and  consequently, 
the  lens,  because  of  its  elasticity,  is  flattened  so  that  its 
diameter  (thickness)  is  decreased  and  the  radius  of  curva- 
ture of  the  anterior  surface  is  increased.  In  other  words, 
by  the  traction  of  the  choroid  coat,  the  convexity  of  the  lens 
is  decreased  and  the  refractive  power  is  therefore  also  de- 
creased.    This  is  the  condition  of  the  lens  when  the  eye  is 


THE     PHYSIOI>OGY    OF    VISION. 


45 


at  rest  and  the  emmetropic  eye  is  then  able  to  see  far  ob- 
jects. 

In  the  anterior  portion  of  the  choroid  we  find  a  small 
group  of  muscles,  called  the  ciliary  muscles.  Some  of  the 
fibres  of  these  muscles  arc  attached  anteriorly  to  the  scler- 


-— ?voce»s«a 


'•V — ^VvoTo'xA 


.  $c\evo\\c 


•^<i^\^\a 


0^i^^^<-^^^■^«■ 


Fig.  14,    Diagrammatic  section  of  the  left  eye. 


Otic  near  its  union  wiih  the  cornea,  at  F,  Fig.  15,  and  pos- 
teriorly they  lose  themselves  in  the  choroid.  These  fibres 
run  parallel  with  the  sclerotic  and  are  called  the  meridional 
fibres  of  the  ciliary  muscles.  Helmholtz  regards  the  an- 
terior end  of  these  muscles  as  fixed.  Now,  when  a  muscle 
contracts  the  two  ends  of  the  muscle  approach  each  other ; 


146 


THE    PHYSIOLOGY    OF    VISION. 


in  fact,  this  is  what  we  mean  by  the  contraction  of  a  muscle. 
When  the  meridional  fibres  of  the  ciliary  muscles  contract, 
the  end  G,  Fig.  15,  approaches  the  fixed  end  F,  and  as  the 
end  G  is  imbedded  in  the  choroid  coat,  the  choroid  coat  is 
drawn  forward  and  the  suspensory  ligaments  are  thereby 
relaxed.  The  lens,  as  we  have  stated,  is  an  elastic  body  and 
if  the  pressure  to  which  it  is  subjected  is  the  same  on  all  sides 


Givovoia 


Fig.  15.    Diagram  sliovving  position  of  the  ciliary  muscles. 
(After  Donders.) 


it  assumes  a  spherical  form.  Consequently,  when  the  trac- 
tion of  the  suspensory  ligaments  is  removed,  the  lens  by  its 
elasticity  assumes  a  more  spherical  form,  so  that  the  radius 
of  curvature  of  the  anterior  surface  is  decreased  from  ten 
millimeters  to  six  millimeters.  At  the  same  time  the  diam- 
eter of  the  lens  increases  and  the  anterior  surface  is  brought 
nearer  to  the  cornea.  The  changes  here  described  are  illus- 
trated in  Fig.  16. 

The  theory  of  accommodation  here  given  is  known  as  the 
Helmholtz  theory,  and  while  it  is  not  the  only  theory  of  ac- 
commodation, it  is  perhaps  the  best.  In  support  of  this 
theory,  we  might  mention  the  following  facts.     If  a  pin  be 


THE    PHYSIOLOGY    OF    VISION. 


47 


stuck  in  the  eye,  so  as  to  pierce  the  sclerotic, the  choroid,  and 
the  retina,  and  the  animal  then  accommodates,  the  head  of 
the  pin  moves  backward;  consequently,  the  internal  portion 
of  the  pin  must  have  moved  forward.  This  would  happen 
if  the  choroid  is  drawn  forward  during  accommodation.  It 
has  also  been  observed  that  if  a  piece  of  the  sclerotic  coat  is 
cut  away  so  as  to  form  a  little  window  in  the  eye,  the 
choroid  can  actually  be  seen  to  move  forward  during  accom- 
modation.    This  has  also  been  observed  in  the  human  eye. 


BMg.  16.  Changes  in  tlie  lens  during  accommodation  (after  Helmholtz) ; 
F,  far  vision;  N,  near  vision  ;  C,  ciliary  process;  cm,  ciliary  muscle. 
1,  iris. 


Another  theory  of  accommodation  is  Tscherning's  theory. 
According  to  Tscherning,  the  lens  is  stretched  when  we 
accommodate  for  a  near  point  and  is  relaxed  during  far 
vision.  It  will  be  seen  that  this  is  diametrically  the  opposite 
of  the  Helmholtz  theory.  Can  it  be  determined  whether 
the  lens  is  relaxed  during  far  or  near  vision?  I  think  that 
by  means  of  the  following  experiment  this  question  can  be 
answered  in  favor  of  the  Helmholtz  theory. 

Experiment  2.  Determine  the  near  point  for  your  eye. 
If  no  other  means  are  at  hand,  this  can  readily  be  done  as 
follows:  Upon  a  foot  ruler  place  a  cork  having  a  groove 
so  that  it  can  ^  slide  along  the  ruler,  and  stick  a  small  pin 
into  this  cork.  Now  place  the  end  of  the  ruler  against 
the    face,   just  beneath   the   eye,   bring   the  cork  nearer  to 


148  THE    PHYSIOLOGY    OF    VISION'. 

the  eye,  and  determine  the  nearest  point  at  which  the  pin  can 
be  seen  distinctly.  Note  how  many  inches  this  is  from  the 
face.  Now  bend  the  head  forward  and  determine  the  near- 
est point  visible.  Note  the  distance.  Next  bend  the  head 
backward  and  again  determine  the  near  point. 

On  making  this  experiment  carefully,  it  will  be  found  that 
the  near  point  in  the  latter  case  is  farther  removed  from  the 
eye  than  in  the  first  case.  The  reason  for  this  is  as  follows : 
During  accommodation  for  the  near  point,  the  lens  is  no 
longer  stretched  by  the  suspensory  ligaments  and  is  there- 
fore free  to  obey  the  laws  of  gravity.     When  the  head  is 


Fig.  17.  Diajirani  showing  position  of  lens  during  accommodation  when 
the  head  is  bent  forward  (outline  of  lens  in  full)  and  when  the  head 
is  bent  backward  (lens  drawn  in  broken  line).  In  the  lirst  case  the 
focus  of  the  near  point  N  falls  on  the  retina  (at  F).  In  the  latter 
case  it  falls  behind  the  retina  at  A,  hence  is  not  seen  distinctly. 

bent  forward,  the  lens  falls  toward  the  cornea,  which  in- 
creases the  distance  between  the  lens  and  the  retina;  hence 
a  nearer  point  can  be  observed  than  when  the  head  is  bent 
backward,  in  which  case  the  lens  falls  toward  the  retina. 
See  Fig.  17. 

A  great  objection  made  against  the  theory  of  Helmholtz 
is  that  in  myopes  the  circular  and  not  the  meridional  fibres 
of  the  ciliary  muscles  undergo  atrophy;  in  hypermetropes 
the  circular  fibres  are  hypertrophied.  This  would  seem  to 
indicate  that  the  circular  fibres  are  of  more  importance  than 
the  meridional  fibres,  which  cannot  be  explained  satisfactory 
by  the  theory  of  Helmholtz. 

Accommodation,  as  we  have  seen,  depends  on  the  elastic- 
ity of  the  crystalline  lens.     With  advancing  age  this  prop- 


THE    PHYSIOLOGY    OF     VISION.  I49 

erty  of  the  lens  becomes  less,  so  that  finally  the  lens  changes 
its  shape  no  longer.  In  this  condition,  called  presbyopia,  or 
old-sightedness,  the  near  point  gradually  recedes,  as  can  be 
seen  from  the  following  table : 

Age.  Distance  of  near  point. 

10  years 7     cm.     or     2.76  inches. 

20  years 10    cm.     or     3.94  inches. 

30  years 14    cm.     cr    5.61  inches. 

40  years 22    cm.     or    8.66  inches. 

50  years 40    cm.    or  15.75  inches. 

60  years i  meter  or    39.37  inches. 

70  years 4  meter  or  157.48  inches. 

It  is  evident  that  if  this  is  the  only  defect  in  the  eye,  it 
can  be  remedied  by  the  use  of  a  convex  lens  used  for  near 
work. 

Two  common  defects  of  vision  are  myopia  and  hyper- 
metropia.  In  myopia,  or  short-sight,  the  eyeball  is  too  long 
and  the  posterior  principal  focus  (of  parallel  rays)  lies  in 
front  of  the  retina.  Hence  the  resting  myopic  eye  does  not 
see  far  but  relatively  near  objects,  these  having  the  Images 
on  the  retina.  This  defect  is  corrected  by  a  concave  lens 
which  delays  the  focussing  of  the  rays. 

The  opposite  condition,  in  which  the  eyeball  is  too  short, 
obtains  in  hypermetropia,  or  long  sight ;  in  this  the  pos- 
terior principal  focus  lies  back  of  the  retina  and  distant  ob- 
jects are  not  seen  distinctly  if  the  eye  is  at  rest.  To  bring 
the  focus  on  to  the  retina  a  convex  lens  is  used.  While  the 
far  and  near  point  for  the  emmetropic  eye  are  infinity  and 
about  six  inches  respectively,  in  myopia  these  points  lie 
nearer  to  the  eye,  while  in  hypermetropia  they  are  further 
removed. 

A  defect  said  to  exist  in  all  eyes  is  astigmatism.  In  the 
theoretical  eye  that  we  have  discussed  in  this  lecture,  the  re- 
fracting surfaces  are  supposed  to  be  segments  of  spheres ; 
in  other  words,  the  various  meridians  of  each  surface  have 


150  THE    PHYSIOLOGY    OF    VISION. 

the  same  radii  of  curvature.  In  reality  this  condition  never 
obtains,  but  the  cornea,  for  example,  has  a  shape  somewhat 
like  the  back  of  a  spoon.  It  is  evident  that  the  light  will 
be  bent  most  along  the  meridian  which  has  the  shortest 
radius  of  curvature  and  therefore  a  luminous  point  will  not 
have  as  its  image  a  point  but  a  figure  more  or  less  like  an 
ellipse.  If  the  difference  in  the  radii  of  curvature  is  great 
(greater  than  one  dioptet"),  the  astigmatism  is  patho- 
logical. In  this  condition  the  person  is  unable  to  see 
horizontal  and  vertical  lines  distinctly  at  the  same  time ;  this 
seriously  interferes  with  distinct  vision  and  must  be  cor- 
rected by  cylindrical  concave  or  convex  glasses. 

It  has  been  claimed  by  some  that  unequal  accommodation 
for  the  two  eyes  is  possible.  In  other  words,  that  we  can 
accommodate  more  with  one  eye  than  with  the  other.  Sci- 
entific experiments  with  the  stereoscope  have  proved,  how- 
ever, that  this  is  impossible ;  the  two  eyes  always  accom- 
modate to  the  same  extent.  It  is  also  held  by  some  that  the 
various  portions  of  the  lens  can  be  brought  into  various 
states  of  accommodation  (astigmatic  accommodation).  By 
this  means  it  would  be  possible  to  obviate  the  indistinct  vis- 
ion of  astigmatism,  for  by  astigmatic  accommodation  the 
astigmatic  patient  could  produce  deformities  in  the  lens  of 
such  a  nature  that  the  original  astigmatism  would  be  abol- 
ished. The  results  of  the  latest  investigation  are  contrary 
to  this  view. 


LECTURE  III. 

At  the  beginning  of  the  previous  lecture,  we  stated  that 
one  of  the  necessary  conditions  for  vision  is  the  formation 
of  a  distinct  image  of  the  object  on  the  retina.  It  is  a 
well-known  fact  that  the  posterior  principal  focus  of  a  per- 
fectly homogenous  spherical  lens  is  not  a  point,  but  a  line. 
This  is  due  to  the  fact  that  peripheral  rays,  A  and  A,  Fig. 
1 8,  come  to  a  focus  sooner  than  the  central  rays,  C  and  C. 

A 


Fig.  18.     Diagram  iUiistrating  spherical  aberration. 

This  is  called  spherical  aberration.  It  is  evident  that  in  this 
condition  the  image  cast  upon  the  screen  or  retina  is  not 
distinct,  for  the  focus  of  the  central  rays  at  C  will  be  sur- 
rounded by  circles  of  diffusion  caused  by  the  peripheral 
rays.  Suppose  that  the  object  from  which  the  rays  emanate 
is  a  point ;  its  focus  will  not  be  a  point  but  a  circle,  the  size 
of  which  varies  with  the  amount  of  spherical  aberration  of 
the  lens.  Hence  the  image  of  the  luminous  point  is  blurred. 
This  defect  is  remedied  in  our  eye  in  perhaps  two  or  three 
ways,  but  the  important  one  is  by  means  of  the  iris. 

Experiment  3.  Look  into  the  eye  of  another  person  and 
observe  the  size  of  the  pupil.  When  this  person  looks  at  a 
distant  object  the  pupil  is  large;  on  accommodating  for  a 
near  point  the  pupil  becomes  much  smaller. 

151 


152  THE    PHYSIOLOGY    OF    VISION. 

This  experiment  indicates  that  the  size  of  the  pupil  varies 
and  is  smaller  for  near  vision.  The  iris  always  cuts  off 
some  of  the  peripheral  rays  which  by  coming  to  a  focus 
sooner  than  the  central  rays  would  cause  blurring  of  the 
image  on  the  retina,  but  it  is  especially  during  near  vision 
that  the  iris  is  most  constricted.  The  object  of  this  is  as 
follows :  The  nearer  the  luminous  point  approaches  the 
eye,  the  greater  is  the  distance  between  the  focus  of  the 
central  and  peripheral  rays ;  in  other  words,  the  greater  the 
divergency  of  the  rays,  the  greater  the  spherical  aberration 
and  the  greater  the  need  for  a  small  pupil.  As  near  vision 
is  always  associated  with  more  divergent  rays,  near  vision 
must  also  be  accompanied  by  pupil  constriction  in  order  to 
produce  a  clear  image.  The  human  eye  is  so  constructed 
that  it  is  impossible  for  us  to  accommodate  without  causing 
pupil  constriction  at  the  same  time. 

Experiment  4.  Look  into  the  right  eye  of  a  person  and 
have  the  left  eye  covered.  Notice  the  size  of  the  pupil  of 
the  open  eye ;  now  uncover  the  left  eye  and  notice  that  the 
pupil  of  the  right  eye  constricts. 

Experiment  5.  Observe  the  pupil  of  a  person  in  very 
dimly  lighted  room ;  now  let  him  walk  into  a  bright  light 
and  the  pupil  will  be  seen  to  constrict.  On  re-entering  the 
darker  room,  the  pupil  dilates. 

These  experiments  teach  that  the  size  of  the  pupil  de- 
pends upon  the  amount  of  light  entering  the  eye.  Besides 
this,  there  are  other  conditions  in  which  the  pupil  of  the 
eye  is  constricted  or  dilated,  which  we  may  summarize  as 
follows:  Constriction  of  the  pupil  is  brought  about  by 
bright  light,  near  vision,  convergence  of  the  eyes,  sleep, 
and  certain  drugs;  dilation  of  the  pupil  is  brought  about 
by  dim  light,  far  vision,  less  convergence  of  the  eyes,  pain, 
fright,  dyspnoea,  and  certain  drugs. 

The  pupil  dilation  and  constriction,  brought  about  by 
light,  is  said  to  be  a  reflex  action,  and  in  order  to  under- 
stand this  phenomenon,  it  is  necessary  that  we  know  what 


THE    PHYSIOLOGY    OF    VISION.  I53 

is  meant  in  general  by  the  term  reflex  action.  The  amoeba, 
Fig.  19,  is  a  lowly  organized  animal  found  in  pools  of  water, 
and  is  microscopic  in  size.  In  the  amoeba  there  is  no  difff^,r- 
entiation  of  tissue,  such  as  we  find  in  the  more  highly  or- 
ganized animals,  the  whole  body  being  composed  of  only 
one  material,  the  living  tissue,  called  protoplasm.  If  the 
amoeba  is  stuck  wnth  a  pin  at  A,  Fig.  19,  it  responds  to  this 
change  in  its  surroundings  by  moving  away  from  the 
point   A.     Instead  of  applying  a  pin   to   the  body  of  the 


a- -- 


Fig.  19 

amoeba,  we  might  have  applied  heat,  an  electrical  shock,  or 
a  chemical  agent ;  all  these  agencies  are  changes  in  the  en- 
vironment of  the  animal,  and  are  called  stimuli.  On  the 
application  of  a  stimulus  the  animal  responds  to  it  by  some 
change  in  its  body ;  this  power  to  respond  to  a  stimulus  is 
called  irritability. 

In  the  amoeba  the  part  of  the  body  which  receives  the 
stimulus  and  the  part  which  responds  to  the  stimulus  are 
similar  in  structure ;  in  fact,  the  part  receiving  the  stimulus 
may  also  respond  to  it.  In  this  organism  there  is  no  physi- 
ological differentiation,  that  is,  there  is  no  division  of  labor. 
In  the  more  highly  developed  organisms,  like  the  human 
body,  the  responding  and  receiving  organ  are  not  one  and 
the  same  organ,  nor  are  they  necessarily  located  in  the  same 
part  of  the  body.     For  example,  when  I  touch  a  hot  object 


154 


THE    PHYSIOLOGY    OF    VISION. 


with  my  finger,  I  withdraw  my  hand.  In  this  case  the  re- 
ceiving organs  are  the  nerve  endings  in  the  skin,  while 
the  responding  organs  are  the  muscles  in  the  arm.  In 
order  that  the  reception  of  the  stimulus  in  the  receiving 
organ  shall  call  forth  a  response  in  the  muscles,  a  connec- 
tion must  exist  between  these  two  organs.  This  connec- 
tion is  formed  by  nerves.  The  physiological  functions  of 
nerves  are  irritability  and  conductivity,  by  which  the  effect 
of  the  stimulus  on  the  receiving  organs  (nerve  endings) 
is  conducted  to  the  muscles. 


Fig.  20.  Scheme  of  reflex  arc.  O  P  is  part  of  the  spinal  cord  ;  S  and  M 
are  sensory  and  motor  nerves  respectively:  A,  skin;  E,  muscle; 
C,  centre  in  cord  where  sensory  and  motor  nerves  come  in  contact. 


This  relationship  between  the  receiving  and  the  respond- 
ing organs  can  be  gathered  from  Fig.  20.  In  this  figure  let 
A  be  a  piece  of  skin  m  which  the  nerve  S  has  its  endings. 
This  nerve  goes  to  the  central  nervous  system,  that  is,  to 
the  spinal  cord  or  to  the  brain.  In  the  cord  or  brain  this 
nerve  ends  and  its  endings  come  in  close  contact  with  the 
endings  of  another  nerve  (at  C,  in  Fig.  20),  which  leaves 
the  spinal  cord  and  supplies  the  muscle,  E.  The  stimulus 
applied  at  A  generates  a  nerve-impulse  which  is  carried  by 
the  nerve  S  to  the  spinal  cord,  leaves  the  cord  by  the  nerve 
M  and  is  carried  to  the  muscle,  causing  this  to  contract. 


THE    PHYSIOLOGY    OF    VISION.  1 55 

Such  an  action  is  called  a  reflex  action.  The  nerve  S 
which  carries  the  nerve  impulse  to  the  central  nervous  sys- 
tem is  called  a  sensory,  or  centripetal,  nerve ;  the  nerve 
M  which  carries  the  impulse  from  the  central  nervous  sys- 
tem to  the  responding  organ,  as  a  muscle,  is  called  a  motor 
nerve,  or  better,  centrifugal  nerve.  The  spot  in  the  central 
nervous  system  where  the  endings  of  the  sensory  and  motor 
nerve  come  in  contact  is  called  a  reflex  center.  In  order 
that  a  reflex  action  shall  take  place,  it  is  evident  that  both 
sensory  and  motor  nerves  must  be  intact. 

For  a  reflex  action,  the  will  or  consciousness  is  not  neces- 
sary, for  many  reflex  actions  take  place  during  sleep.  To 
show  this  still  more  plainly  we  may  take  the  following. 
When  a  frog  is  decapitated,  it  certainly  has  lost  all  psychical 
functions,  such  as  memory,  consciousness,  and  will,  granting 
that  the  frog  had  these  faculties  before  it  was  decapitated. 
If  such  a  decapitated  frog  is  suspended  and  a  bit  of  blotting 
paper  soaked  with  an  acid  is  placed  on  its  thigh,  the  frog 
will  draw  up  one  of  its  hind  legs  and  wipe  away  the  irri- 
tating substance,  definitely  locating  it  on  its  body.  Hence 
this  action,  which  is  true  reflex  action,  is  produced  without 
the  intervention  of  any  psychical  function.  This  action  is 
rendered  impossible  if  either  the  sensory  nerve  leading  from 
the  skin  or  the  motor  nerve  supplying  the  muscle  is  cut.  If 
the  spinal  cord  is  destroyed,  the  action  also  stops,  because 
the  connection  between  sensory  and  motor  nerves  is  there- 
by destroyed,  and  the  impulse  sent  by  the  sensory  nerves 
cannot  be  transferred  to  the  motor  nerve. 

Now  the  pupil  constriction  which  takes  place  during 
bright  light  is  similar  in  nature  to  the  actions  here  de- 
scribed. It  is  a  true  reflex  action  and  can  take  place  even 
in  opposition  to  the  will,  for  we  cannot  by  the  exercise  of 
this  faculty  prevent  the  constriction  of  the  pupil. 

The  next  question,  then,  is  to  determine  the  pathway  of 
the  impulse  producing  this  action,  in  other  words,  to  deter- 
mine  the   sensory   and   motor   nerves   concerned    in   pupil 


156  THE    PHYSIOLOGY    OF    VISION. 

constriction.  It  has  been  found  that  if  the  retina,  in  which 
the  sensory  nerve  endings  of  tlie  optic  nerve  are  situated,  is 
diseased,  or  if  the  optic  nerve,  the  second  cranial  nerve,  is 
cut,  the  constriction  of  the  pupil  in  bright  light  no  longer 
takes  place.  Consequently  we  regard  the  optic  nerve  as  the 
sensory  nerve  of  this  pupil  reflex.  Again,  it  has  been  found 
that  if  the  third  cranial  nerve,  the  oculo-motor  nerve,  is  cut, 
the  pupil  reflex  also  disappears,  while  if  the  peripheral  end 
of  the  cut  third  cranial  nerve,  i.  e.,  the  end  attached  to  the 
eye,  is  stimulated,  constriction  follows.  From  this  it  is  ap- 
parent that  the  sensory  and  motor  nerves  of  the  pupil  reflex 
are  the  second  and  third  cranial  nerves  respectively. 

The  pupil  reflex  has  been  used  as  a  means  to  determine 
blindness.  If  one  eye  is  blind  to  light,  no  pupil  constriction 
takes  place  in  this  eye  when  a  bright  light  is  cast  into  it. 
But  in  making  this  experiment,  it  is  absolutely  necessary  to 
exclude  the  light  from  the  sound  eye,  for  it  has  been  found 
that  when  a  light  is  cast  into  one  eye,  the  pupil  of  the  other 
eye  also  constricts,  even  though  it  be  in  darkness.  This  is 
known  as  consensual  pupil  reflex  and  is  due  to  the  partial 
decussation  of  the  optic  fibers,  as  shown  in  Fig.  21.  Let  L 
E  and  R  E  represent  the  retina  of  the  left  and  right  eye  re- 
spectively. The  external  fibers  (broken  lines  in  Fig.  21) 
from  the  left  eye  proceed  to  the  left  side  of  the  brain, 
L  O,  and  the  external  fibers  of  the  right  eye  proceed  to  the 
right  side  of  the  brain,  R  O;  but  the  internal  fibers  (full 
lines  in  Fig.  21)  of  the  left  eye  cross  over  to  the  right  side, 
while  the  internal  fibers  of  the  right  eye  cross  over  to  the 
left  side  of  the  brain.  Consequently,  light  falling  upon  the 
left  eye  affects  not  only  the  left  but  also  the  right  side 
of  the  brain  and  the  nerve  impulse  is  carried  to  the  pupil 
constricting  center  for  each  eye  and  from  thence  is 
sent  to  both  irises.  The  constricting  center  is  situ- 
ated in  the  anterior  corpora  quadrigemina  of  the  brain. 

We  must  now  discuss  the  responding  organ  in  the  iris 
itself.    What  structure  in  the  iris  causes  constriction  or  dila- 


THE    PHYSIOLOGY    OF     VISION. 


57 


tion  upon  stimulation  of  the  retina  by  light?  It  was  stated 
in  the  second  lecture  that  the  iris  is  a  continuation  of  the 
choroid.     In  the  iris  are  found  two  sets  of  muscles;    the 


Fig.  21.  Diajfrani  illustrating  the  decussation  of  optic  nerves;  LE,  left 
eye;  RE,  right  eye;  LO  left  cerebral  hemisphere;  KO  right  cerebral 
hemisphere;  C,  chiasma. 


tibers  of  one  set  having  a  circular  course  (parallel  with  the 
pupil),  the  others  have  a  radial  course,  extending  from  the 
periphery  towards  the  central  part  of  the  iris.  These  are 
called  the  circular  and  the  radial  fibers  of  the  iris.  When 
the  circular  fibers  contract,  the  pupil  constricts ;   contraction 


158  THE    PHYSIOLOGY    0¥    VISION. 

of  the  radial  fibers  causes  dilation  of  pupil.  The  third 
cranial  nerve  governs  the  circular  fibers  and  consequently 
stimulation  of  this  nerve  is  followed  by  the  contraction 
of  the  circular  fibers,  which  causes  the  pupil  to  become 
smaller.  The  radial  fibers  of  the  iris  are  under  the  influence 
of  the  cervical  sympathetic  nerve  and  painful  stimulation 
of  almost  any  part  of  the  body  is  followed  by  a  dilation  of 
the  pupil,  the  impulse  being  carried  to  the  pupil  over  the 
sympathetic  nerve.  Mental  conditions,  as  I  have  already 
stated,  also  influence  the  pupil;  fright,  for  example,  causes 
dilation. 

Among  the  many  drugs  which  influence  the  pupil,  we 
may  first  of  all  mention  the  myotics,  such  as  opium,  ether, 
and  physostigmin  or  eserin,  which  cause  constriction  of  the 
pupil.  Among  the  drugs  causing  dilation  of  the  pupil  are 
atropin  and  cocaine;  these  drugs  are  called  mydriatics. 
Atropm  causes  dilation  of  the  pupil  by  paralyzing  the  end- 
ings of  the  third  cranial  nerve  and  consequently  light  falling 
into  an  eye  treated  with  this  drug  causes  no  constriction. 
It  must  also  be  borne  in  mind  that  atropin  causes  paralysis 
of  the  muscles  of  accommodation  for  the  same  reason,  hence 
an  eye  treated  with  atropin  cannot  accommodate  for  near 
objects.  We  may  also  mention  that  alcohol  dilates  and 
morphine  constricts  the  pupil. 

We  may  still  refer  to  the  curious  phenomenon  of  the 
rythmical  dilation  and  constriction  of  the  pupil  depending 
upon  the  heart  beat.  If  you  attentively  observe  the  pupil  of 
your  neighbor's  eye,  you  may  see  very  limited  constrictions 
and  dilations  following  each  other  and  coinciding  with  the 
heart  beat,  as  can  be  ascertained  by  feeling  his  pulse.  This 
is  observed  better  if  the  pupil  is  magnified  by  means  of  a 
convex  lens. 

The  constriction  and  the  dilation  of  the  pupil  can  be  seen 
entoptically  in  one's-  own  eye.  By  entoptical  perception  we 
mean  seeing  objects  located  in  the  eye  itself.  A  great 
variety  of  things  can  be  seen  in  the  eye  by  the  person  him- 


THE    PHYSIOLOGY     OF    VISION, 


^^9 


self,  and  the  principle  upon  which  most  of  these  entoptical 
visions  are  based  is  as  follows.  In  Fig.  22,  let  the  eye  be 
emmetropic  and  the  muscles  of  accommodation  relaxed  as 
in  far  vision.  In  the  plane  of  the  anterior  principal  focus, 
which  is  located  about  one-half  inch  in  front  of  the  eye,  is 
placed  a  cardboard  with  a  small  pinhole.  The  daylight 
streaming  through  this  pinhole  leaves  it  divergingly,  the 
result  being  the  same  as  if  the  source  of  light  was  situated 
in  the  pinhole,  that  is,  in  the  anterior  principal  focus  of  the 
eye.  If  the  light  is  situated  in  the  anterior  principal  focus 
of  an  optical  system,  the  rays  after  refraction  are  parallel. 


Fig.  22.    IMustrating  entoptical  vision. 


Consequently,  in  Fig.  22  the  rays  after  passing  through  the 
lens  are  parallel  with  the  optical  axis  and  a  certain  portion 
of  the  retina  is  illuminated.  The  size  of  the  luminous  circle 
on  the  retina  is  determined  by  the  size  of  the  pupil.  The 
larger  the  pupil,  the  larger  the  circle ;  the  smaller  the  pupil, 
the  smaller  the  area  of  the  retina  illuminated. 

Experiment  6.  Place  the  cardboard  with  a  pinhole  in 
the  plane  of  the  anterior  principal  focus  of  the  eye  (one- 
half  inch  in  front  of  the  eye)  and  look  through  the  pinhole 
at  the  bright  sky.  A  luminous  circle  is  seen.  Now  intro- 
duce a  pin  from  below  upwards  between  the  cardboard  and 
the  eye,  as  illustrated  in  Fig.  22.  A  shadow  of  the  pin  is 
seen.     This  shadow  is   formed   on  the  lower  part  of  the 


l6o  THE    PllYSIOl.OGY    OF    VISION. 

retina,  but  to  the  experimenter,  the  shadow  appears  to  pro- 
ceed from  above  downward.  The  reason  for  this  reversal 
we  will  learn  later  on,  but  we  may  here  state  that,  as  the  eye 
behaves  similar  to  a  simple  convex  lens,  the  images  on  the 
retina  are  alwriys  inverted. 


LECTURE   IV. 
In  the  last  lecture  we  discussed  the  principle  upon  which 
most  entoptical  perceptions  are  based.     By  placing  a  card- 
board with  a  pinhole  in  the  anterior  principal  focus,  a  lumin- 
ous circle  is  cast  upon  the  retina,  and  an  opaque  body  situ- 


Fig.  23.    Striae  produced  by  winking  the  eyelids   (after  George  Bui  ) 

ated  between  the  pinhole  and  the  retina  casts  a  shadow 
upon  the  retina  which  can  be  perceived.  In  this  manner 
we  observed  the  shadow  of  a  pin  placed  between  the  eye  and 
the  cardboard  (see  Exp.  6  and  Fig.  22).  But  in  entoptical 
perception  the  bodies  seen  are  situated  in  the  eye  itself. 

161 


l62  THE    PHYSIOLOGY    OP^    VISION. 

Experiment  7.  While  the  cardboard  with  the  pinhole  is 
in  the  anterior  principal  focus,  and  the  eye  looks  through 
the  hole  at  the  bright  sky,  close  the  eyelid  and  you  will  see 
a  field  somewhat  like  that  pictured  in  Fig.  23.  The  hori- 
zontal striations  are  due  to  the  fact  that  in  pressing  the  eye- 
lid downward,  you  have  collected  the  tears  and  they  reflect 
the  light  and  center  it  more  on  the  retina.  It  has  also  been 
suggested  that  the  striations  are  due  to  wrinkles  of  the 
epithelial  layer  of  the  cornea  caused  by  the  movements  of 
the  lid. 

Another  phenomenon  that  can  be  observed  entoptically  is 
the  changes  in  the  size  of  the  pupil  during  accommodation. 

Experiment  8.  As  in  the  previous  experiment  look  at 
the  sky  through  the  pinhole.  "  The  luminous  circle  has  a 
certain  size.  Now  place  a  pin  about  eight  or  ten  inches  in 
front  of  the  card  and  fix  your  vision  upon  this.  While 
doing  so,  the  luminous  field  becomes  smaller.  Again  relax 
your  accommodation,  i.  e.,  look  at  the  sky,  and  the  circle  in- 
creases in  size. 

This  change  in  the  size  of  the  luminous  circle  is  due  to 
the  constriction  of  the  pupil  which  accompanies  accommo- 
dation for  a  near  point.  The  constriction  of  the  pupil  in 
bright  light  (see  Lecture  III)  can  also  be  observed  en- 
toptically. 

Experiment  9.  Face  a  well-lighted  window,  close  one 
eye  and  look  through  the  pinhole  with  the  other  eye.  The 
luminous  field  has  a  certain  size.  Now  open  the  other  eye ; 
the  size  of  the  circle  becomes  smaller,  due  to  the  con- 
striction of  the  pupil. 

In  a  similar  manner,  the  condition  of  the  crystalline  lens 
can  be  studied. 

Experiment  10.  Use  a  very  small  pinhole,  place  the  card 
in  the  anterior  principal  focus,  and  look  at  a  uniform  sur- 
face, as  the  sky.  A  grayish  field  with  a  star-shaped  figure 
(Fig.  24)  will  be  seen.  Scattered  here  and  there  you  will 
also  observe  small  spherical  bodies,  some  lighter  and  some 


THE    PHYSIOLOGY    OF    VISION. 


163 


darker  than  the  field.  The  stellate  figure  is  due  to  the 
cement  substance  of  the  lens.  The  crystalline  lens,  as  you 
may  know,  is  composed  of  a  number  of  layers,  somewhat 


Fig.  24.    Diagram  representing  half  of  the  lens  as  seen  entoptically. 
(After  Donderf.) 


like  an  onion ;  these  layers,  however,  do  not  form  com- 
plete semi-circles  but  have  a  course  which  can  best  be 
understood  from  Fig.  25,  The  ends  of  the  layers  are  joined 
by  a  cement-like  substance,  and  it  is  this  substance  that 
gives  rise  to  the  stellate  figure  seen  in  Exp.  10.  The  light 
and  dark  spot  seen  in  Exp.  10  are  due  to  small  imperfec- 
tions of  the  lens  which  have  been  observed  microscopic- 
ally in  the  lens.  If  the  lens  were  perfectly  homogeneous, 
i.  e.,  composed  of  one  material,  you  would  not  observe  these 
things.  As  a  person  grows  older,  these  imperfections  of  the 
lens  increase  and  the  stellate  figure  may  become  black ; 
this  may  in  part  be  the  cause  of  the  diminished  acuteness 
of  vision  in  old  age. 

A  group  of  bodies  in  the  eye  most  frequently  observed 
are  the  muscae-volitantes,  or  flies.  These  muscae-voli- 
tantes  can  be  readily  seen  by  looking  at  the  bright  sky,  but 
by  means  of  the  following  experiment  they  become  more 
distinct. 

Experiment  it.  IMace  the  cardboard,  as  in  Exp.  to,  but 
use  a  large  hole.     Look  at  the  sky  and  you  will  see  scat- 


164 


THE    PHYSIOLOGY    OF    VISION. 


tered  over  the  field  little  disks.  Sometimes  you  will  see 
them  as  single  disks,  sometimes  they  are  in  strings  or 
groups,  Fig.  26.  If  you  fix  a  certain  point  in  the  sky,  you 
will  notice  that  the  muscae  are  always  floating  downward. 
Try  to  fix  your  line  of  vision  upon  one,  and  it  evades  you ; 
for  this  reason  they  are  called  muscae-volitantes,  or  fiying 
flies. 


Fig.  25.  A,  Laminated  structure  of  the  crystalline  lens,  showing  the 
denser  nucleus  and  the  concentric  outer  layers.  B,  Diagram  show 
ing  arrangement  of  lens  fibres  (posterior  view.) 

Ordinarily  we  do  not  see  these  flies,  but  when  the  light 
enters  our  eye  in  such  a  manner  that  these  bodies  cast  a 
strong  shadow  upon  the  retina,  we  become  conscious  of 
them.  Some  of  these  muscae  are  very  permanent.  Natur- 
ally you  ask  what  are  these  bodies.  Bonders  found  that 
these  muscae-volitantes  are  imperfections  in  the  vitreous 
humor,  which  have  a  greater  or  a  lesser  refracting  power 
than  the  vitreous  humor  itself.  They  are  said  to  become 
more  numerous  in  myopes,  some  myopes  being  so  troubled 
with  them  that  the  anxiety  caused  by  them  becomes  a  case 
of  monomania. 

Still  another  phenomenon  that  can  be  seen  entoptically  is 
the  phosphene. 

Experiment  12.  With  the  tip  of  the  finger  press  the 
corner  of  the  eyeball.  Notice  in  the  opposite  side  of  the 
visual  field  a  dark  disk  surrounded  by  a  very  light  band. 
That  which  is  seen  when  the  cornea  is  thus  pressed  is  called 


THE    PHYSIOLOGY    OF    VISION.  105 

a  phosphene,  and  is  caused  by  the  mechanical  stimulation 
of  the  retina.  Something  similar  to  this  can  be  seen  by  rub- 
bing the  closed  eyes  with  the  knuckles.  A  most  beautiful 
display  of  colors  in  ever  varying  pattern,  very  much  like 
the  field  seen  in  a  kaleidoscope,  presents  itself. 

Besides  these,  we  can  also  observe  the  blood-vessels  in 
our  own  eye.  In  a  certain  layer  of  the  retina  are  located 
the  blood-vessels  which  supply  the  retina,  and  if  the  light 
falls  into  the  eye  in  a  strange  fashion  the  shadows  of  these 
vessels  are  seen. 


.  Fig.  26.    Muscae  Volitantes. 

Experiment  13.  In  a  dark  room  hold  a  candle  quite 
close  to  the  eye,  and  a  little  to  the  right,  when  you  are  ex- 
perimenting with  the  right  eye.  By  moving  the  candle  to 
and  fro  you  will  see  depicted  upon  an  orange  field  a  beau- 
tiful display  of  blue  lines  which  branch  again  and  again. 

These  are  the  blood-vessels.  They  always  cast  shadows 
on  the  retina  and  yet  we  are  never  conscious  of  them. 
Why  we  do  not  see  them  ordinarily  is  perhaps  difficult  to 
explain,  but  we  may  say  that  what  we  always  see  we  never 
see,  or,  more  correctly,  images  that  are  constantly  on  the 
retina  (like  the  shadows  of  these  blood-vessels)  no  longer 
affect  our  consciousness.  A  stimulus  is  a  change  in  our 
environment;  we  are  only  conscious  of  changes.  Hence, 
when  the  position  of  the  shadows  of  the  retinal  vessels  is 
changed,  then,  and  then  only,  do  we  perceive  them.  This 
is  accomplished  by  letting  the  light  enter  the  eye  from  a 
different  direction  than  it  normally  does. 


l66  THE    PHYSIOLOGY    OF    VISION. 

Not  only  can  the  blood-vessels  of  the  retina  be  seen  en- 
toptically,  but  even  the  blood  streaming  through  these  ves- 
sels can  be  thus  observed. 

Experiment  14.  Hold  a  piece  of  blue  glass  in  front  of 
the  eye  and  look  steadily  at  the  bright  sky.  Numerous 
little  light  spots  will  be  seen  crossing  the  field  in  tortuous 
lines.  The  specks  appear  and  disap])ear  very  suddenly  and 
are  immediately  followed  by  others,  their  motion  reminding 
one  very  much  of  the  skating  of  certain  water-beetles  on  the 
surface  of  the  water.  After  looking  at  them  for  sometime, 
you  can  trace  out  the  definite  paths  they  follow. 

It  is  perhaps  well  known  to  you  that  in  the  blood  we 
find  an  innumerable  number  of  microscopical  disks  called 
red  blood-corpuscles.  Now  the  light  specks  that  you  ob- 
served in  the  previous  experiment  are  these  corpuscles 
floating  in  the  blood  as  it  flows  through  the  retinal  vessels. 
It  is  supposed  that  the  corpuscles  in  flowing  through  the 
retina  stimulate  the  retina  mechanically,  very  much  the 
same  as  you  did  in  Experiment  12,  and  hence  they  appear 
as  luminous  circles. 

Before  we  dismiss  the  subject  of  entoptical  vision,  I  must 
call  your  attention  to  one  more  phenomenon,  closely  re- 
lated to  this  subject.  On  entering  a  perfectly  dark  room, 
you  would  naturally  expect  to  see  absolute  darkness.  If, 
however,  you  stay  in  the  room  until  the  eyes  become  accus- 
tomed to  darkness,  you  will  notice  that  the  field  before  you 
is  not  absolutely  black,  but  has  the  appearance  of  a  faint 
misty  haze  or  glow.  This  is  called  the  intrinsic,  or  spe- 
cific, light  of  the  retina.  By  this  intrinsic  light  of  the 
retina  we  do  not  mean  to  say  that  there  is  actual  light  in 
the  eye;  it  is  a  sensation  of  light,  the  cause  of  which  is  not 
well  understood.  Some  ha^'c  supposed  that  it  is  due  to  the 
bombardment  of  the  sensitive  portion  of  the  retina  by  the 
blood-corpuscles  referred  to  in  Experiment  14.  Others 
hold  that  it  is  not  due  to  the  retina,  but  to  changes  in  that 
part  of  the  brain  where  our  visual  sensations  are  produced. 


"HE     rilYSIOI.OGY    OF    VISION. 


167 


It  is  a  well-known  fact  that  in  certain  diseased  conditions, 
as  in  delirium  tremens,  the  visual  centers  of  the  brain  are 
abnormally  stimulated,  and  the  patient  imagines  he  sees 
objects  which  have  no  objective  existence. 


Fig.  a7.  Diagram  of  structure  of  the  Retina  (after  Cajal).  H,  layer  of 
nerve-fibres;  G,  layer  of  ganglion  cells;  F,  internal  molecular  layer; 
E,  internal  nuclear  layer;  C,  external  molecular;  B,  external  nu- 
clear; A,  layer  of  rods  and  cones. 


The  second  requisite  for  distinct  vision  is  the  production 
of  a  change  by  the  light  in  the  eye  of  such  a  nature  that  it 
can  be  communicated  to,  or  affect  the  endings  of,  the  optic 
nerve  in  the  eye.  The  sensitive  clement  of  the  eye  without 
which  sight  is  impossible  is  the  retina,  the  innermost  coat  of 
the  eye  (see  Fig.  14).  The  retina  has  an  exceedingly  com- 
plex structure,  but  as  our  time  is  limited,  wc  shall  only 
refer  to  the  most  important  points. 

The  optic  or  second  cranial  nerve  is  the  nerve  of  sight, 
the  impulses  generated  by  the  light  in  the  eye  are  by  means 


l68  THE     PHYSIOLOGY    OF    VISION. 

of  this  nerve  conveyed  to  the  brain.  This  nerve  arises  from 
various  structures,  such  as  the  geniculate  bodies,  optic 
thalamus,  anterior  corpus  quadrigcminum,  etc.,  found  in 
the  anterior  portion  of  the  brain.  Soon  after  leaving  the 
brain,  the  two  optic  nerves  meet  in  the  median  line  of  the 
brain  and  after  partial  decussation  (see  Fig.  21)  again  sep- 
arate and  proceed  to  the  eyes  and  end  in  the  retina.  But 
the  endings  of  the  fibers  of  the  optic  nerve  in  the  retina 
come  in  contact  (in  layer  F,  Fig.  27)  with  the  endings  of 
another  set  of  very  short  nerves.  The  other  endings  of 
these  short  nerves  again  come  in  contact  with  the  endings 
of  a  third  set  of  short  nerves  (in  layer  C,  Fig.  27),  whose 
external  ends  (upper  ends  in  Fig.  27)  have  a  peculiar 
structure,  known  as  the  rods  and  cones.  Hence  there  are 
three  sets  of  nerves  in  the  retina.  And  it  must  be  borne  in 
mind  that  the  fibers  coming  from  the  brain,  and  which  are 
located  in  the  layer  of  the  retina  designated  H  in  Fig.  27, 
are  innermost  in  the  eye,  while  the  layer  of  rods  and  cones 
(layer  A,  Fig.  27)  is  in  the  external  part  of  the  retina,  that 
is,  it  is  in  contact  with  the  choroid  (see  Fig.  14). 

The  question  is,  which  part  of  this  complicated  mechan- 
ism is  acted  upon  by  light.  In  order  not  to  burden  you  with 
too  many  details,  we  may  say  that  the  rods  and  cones  are  re- 
garded as  the  ultimate  elements  of  sight ;  they  form  the 
structures  that  the  light  acts  upon  in  such  a  manner  as  to 
cause  vision.  Some  of  the  reasons  for  this  supposition  are 
as  follows.  The  portion  of  the  retina  where  the  optic  nerve 
enters  is  known  as  the  optic  disk ;  it  is  also  called  the  blind 
spot,  because  this  part  of  the  retina  is  absolutely  blind 
(see  Fig.  14). 

Experiment  15.  Close  the  left  eye  and  with  right  eye 
look  at  the  cross  in  Fig.  28 ;  by  indirect  vision  you  will 
perceive  the  circle.  Bring  the  page  closer  to  the  eye,  all 
the  time  keeping  your  eye  fixed  on  the  cross.  At  a  certain 
distance  the  circle  will  disappear.  Bring  the  page  still 
nearer  and  the  circle  will  reappear. 


THE    PHYSIOLOGY    OF    VISION.  1 69 

This  is  known  as  Mariotte's  experiment  and  is  explained 
as  follows.  When  we  look  directly  at  an  object,  the  focus 
falls  on  the  fovea  centralis,  or  yellow  spot  (Fig.  14), and  the 
images  of  other  objects  not  in  line  with  the  first  object  fall 
outside  of  the  yellow  spot.  When  the  page  is  held  in  the 
correct  position,  the  image  of  the  cross  falls  on  the  yellow 
spot  and  that  of  the  circle  on  the  blind  spot.  By  micro- 
scopical examination  it  has  been  found  that  there  are  no 
rods  or  cones  in  the  blind  spot,  hence  this  experiment  is  a 
strong  proof  in  favor  of  the  idea,  that  the  rods  and  cones 
are  the  percipient  elements.  Right  here  I  may  draw  your 
attention   to   something  interesting.  •  In  the   optic  disk  or 


+ 


Fig.  28. 


blind  spot  there  is  an  abundance  of  nerve  fibers  for,  as  we 
have  stated  before,  here  the  optic  nerve  enters.  The  light 
falls  upon  there  optic  nerve  fibers  and  yet  there  is  no  sensa- 
tion produced.  We  may  conclude,  therefore,  that  the  nerve 
fibers  themselves  cannot  be  stimulated  by  light;  the  light 
must  fall  upon  the  endings  of  the  nerves  (rods  and  cones) 
in  order  that  a  sensation  of  light  be  produced.  This  is  anal- 
ogous to  other  sense  organs.  In  order  to  have  a  sensation 
of  touch  the  nerve  endings  in  the  skin  must  be  stimulated, 
for  if  the. skin  is  removed,  touching  the  exposed  nerve  may 
give  rise  to  a  sensation  of  pain,  but  not  of  touch. 

I  may  also  state  that  Bonders  was  able,  by  means  of  a 
small  mirror,  to  throw  a  light  on  the  blind  spot,  leaving 
the  rest  of  the  retina  dark.  In  this  case  the  person  was  not 
conscious  of  any  sensation  of  light. 

Another  reason  why  we  are  certain  that  the  rods  and 
cones  are  the  structures  stimulated  by  light  is  as  follows: 
As  I  told  you,  when  we  look  directly  at  an  object,  the 
focus  always  falls  on  the  fovea  centralis,  or  yellow  spot; 


170  THE    PHYSIOLOGY    OF    VISION. 

at  this  portion  of  the  retina  vision  is  keenest.  Now  this 
portion  of  the  retina  is  also  best  suppHed  with  rods  and 
cones,  especially  the  latter.  As  we  proceed  from  the  yellow 
spot  to  the  periphery  of  the  retina,  vision  becomes  less  and 
less  distinct,  and  the  number  of  rods  and  cones,  especially 
cones,  becomes  also  less  and  less. 

That  the  rods  and  cones  are  the  percipient  elements  of 
sight  is  further  rendered  evident  by  the  fact  that  whenever 
they  are  destroyed,  even  if  the  other  portions  of  the  retina 
are  intact,  vision  is  gone.  To  these  proofs  we  may  still 
add  the  following,  which  is,  I  think,  of  interest. 

Experiment  16.     Look  at  the  two  points  printed  beneath. 

•• 

These  two  points  are  seen  as  two  distinct  points.  Move 
away  from  these  points  to  a  sufficient  distance,  and  you  will 
see  only  one  point ;  the  two  points  have  fused  and  only  one 
sensation  is  produced. 

If  the  two  points  are  further  apart  than  here  indicated, 
the  distance  between  your  eyes  and  the  points  must  be 
greater  in  order  to  cause  them  to  fuse.  In  other  words,  it 
is  the  angle  under  which  these  two  points  are  seen  that 
determines  whether  you  see  them  as  two  points  or  as  one 
point.  The  smallest  visual  angle  under  which  two  points 
are  seen  as  two  distinct  points  lies  between  50  and  70  sec- 
onds. The  following  explanation  is  generally  given  for 
this  phenomenon.  In  Fig.  29,  N  is  the  nodal  point  of  the 
eye.  By  the  nodal  point*  we  mean  a  point  in  an  optical  sys- 
tem of  such  a  nature  that  a  ray  of  light  going  -towards  it 
before  refraction,  is  not  refracted  in  going  through  the 
optical  system.  In  Fig.  29,  let  A  and  B  represent  the  two 
points  looked  at.  Each  point  sends  a  ray  of  light  to  the 
nodal  point  and  these  rays  are  focused  on  the  retina  at  A' 
and  B' ;  hence.  A'  is  the  image  of  A,  and  IV  is  the  image 
of  B.  If  the  angle  included  between  the  lines  A  N  and  B  N 
is  73  seconds,  the  linear  distance  between  the  images  A'  and 
B'  is  about  5.36  micromillimeters,  or  .00536  millimeters ;    in 

*In  reality  there  are  two  nodal  points,  but  as  tliey  lie  close  to 
gether,  tliey  may  be  regarded   as  one. 


THE    PHYSIOLOGY    OF    VISION.  I7I 

this  case  the  points  are  seen  separately.  But  suppose  that 
the  points  are  situated  closer  together,  the  angle  A  N  B 
will  be  less  than  73  seconds  and  the  distance  A'  B'  will  be 
less  than  .00536  mm. ;  in  this  instance  the  two  points  are 
seen  as  one  point.  This  is  due  to  the  fact  that  if  two  images 
fall  on  one  rod  or  cone  we  have  but  one  sensation,  while 
if  the  two  images  fall  on  two  rods  or  cones  separated  by  a 
third  cone,  two  distinct  points  are  seen.  In  A  and  B  of 
Fig.  30  the  two  images  fall  either  on  one  cone  or  on  two 
neighboring  cones,  and  only  one  sensation  is  produced.  In 
C,  Fig.  30,  two  cones  separated  by  a  third  cone  are  stimu- 


Fig.  29. 

lated,  and  two  sensations  are  produced.  It  was  stated  that 
the  images  (A'  and  B',  Fig.  29)  must  be  about  .005  mm. 
apart  in  order  to  be  perceived  as  two  points.  x\s  the 
diameter  of  the  cones  is  from  .002  to  .005  mm,  these  facts 
agree  with  the  theory  that  the  rods  and  cones  are  the  ulti- 
mate elements  of  sight.  It  may  be  mentioned  in  passing  that 
the  determination  of  the  visual  actuity  by  the  charts  of 
Snellen,  is  based  to  some  extent  on  this  principle. 

Having  decided  that  it  is  the  rods  and  cones  which  are 
stimulated  by  light,  the  next  step  is  to  determine  what 
change  is  produced  in  the  eye  which  enables  us  to  see. 
There  are  quite  a  number  of  changes  produced  by  the  light 
in  the  eye.  In  the  outer  portions  of  the  rods  (see  Fig.  27) 
is  found  a  reddish  pigment  called  rhodopsin,  or  visual 
purple.  This  pigment  bleaches  when  exposed  to  light  and 
in  darkness  it   regains   its   colors.      It   behaves,   therefore, 


172  THE    PHYSIOLOGY    OF    VISION. 

somewhat  like  a  photographic  plate,  but  is  superior  to  it  in 
that  darkness  restores  the  pigment.  In  fact,  by  means  of 
this  visual  purple  Kuhne  was  able  to  form  a  picture  of  an 
external  object  on  the  retina;  such  a  picture  is  known  as 
an  optogram.  Kuhne  placed  a  frog  in  a  dark  room  for  an 
hour  or  two  so  as  to  increase  the  amount  of  visual  purple. 
He  then  excised  the  eye  and  placed  it  in  front  of  a  window 
so  that  an  image  of  the  window  with  its  panes  and  sashes 
fell  upon  the  retina.  After  an  exposure  of  some  minutes  he 
took  the  retina  out  of  the  eye  and  treated  it  with  an  alum 
solution,  which  ''fixes"  the  rhodopsin  so  that  it  is  no  longer 

000  GXDO   000 

A  B  C 

Fig.  30.  Tn  these  figures  the  two  points  correspond  to  the  images 
a'  and  b'  of  fig.  29.  In  A  they  faU  on  one  cone,  in  B  on  two  neighbor- 
ing cones ;  in  both  cases  one  sensation  is  produced  and  consequently 
one  point  perceived.  In  B  the  two  images  faU  on  two  cones  separ- 
ated bv  a  third  cone  and  this  causes  two  sensations  and  hence  two 
distinct  points  are  perceived. 

affected  by  light.  He  then  observed  a  picture  of  the  win- 
dow on  the  retina,  in  which  the  panes  were  colorless 
(bleached)  and  the  sashes  red. 

When  this  action  of  visual  purple  was  first  discovered,  it 
was  thought  that  the  riddle  of  how  light  produces  its  effect 
in  the  eye  was  solved.  However,  this  fond  illusion  was 
soon  dispelled,  for  it  was  found  that  visual  purple  is  not 
necessary  for  vision,  as  is  indicated  by  the  following  facts. 
There  are  some  animals,  like  snakes  and  pigeons,  that  have 
no  visual  purple ;  as  these  animals  can  see  perfectly  well,  it 
furnishes  good  grounds  for  supposing  that  it  is  not  abso- 
lutely necessary  for  our  vision.  Again,  if  the  human  eye  is 
exposed  to  the  bright  sky  for  ten  or  fifteen  minutes  all  the 
rhodopsin  is  bleached,  notwithstanding  the  eye  is  not  blind. 
Visual  purple  is  not  affected  by  a  red  light,  yet  we  are 
able  to  see  red  light.  Besides  this,  the  changes  that  rhodop- 
sin undergoes  when  exposed  to  light  are  too  slow  to  account 


THE    PHYSIOLOGY    OF    VISION.  1 73 

for  vision;  a  flash  of  lightning  is  of  too  short  a  duration 
to  affect  this  pigment. 

There  are  many  other  changes  produced  in  the  eye,  but 
none  of  them  are  absolutely  essential  to  vision,  so  far  as  we 
know  at  present.  Hence  we  are  ignorant  of  the  second 
requisite  for  vision,  namely,  a  change  produced  by  light  in 
the  eye.  Nor  are  we  better  informed  of  the  third  requi- 
site, the  communication  of  this  (unknown)  change  to  the 
optic  nerve  and  its  propagation  to  the  brain.  We  are  cer- 
tain that  the  light  stimulates  the  endings  of  the  optic  nerve, 
and  that  this  nerve  conducts  a  nerve  impulse  to  the  brain 
which  finally  results  in  conscious  vision;  but  what  this 
nerve  impulse  consists  of  we  cannot  at  present  tell.  Most 
likely  it  is  no  different  from  the  nerve  impulses  going  over 
other  nerves  in  the  body;  the  sensation  of  vision  is  not  de- 
termined by  any  definite  kind  of  stimulation  or  nerve  im- 
pulse reaching  the  brain,  but  upon  the  definite  place  (cen- 
ter) in  the  brain  where  the  nerve  impulse  causes  a  change 
in  the  cells  of  the  brain.  As  we  shall  see  later  on,  the  visual 
sensations  originate  in  the  occipital  lobes  of  the  cerebral 
hemispheres  of  the  brain.  Here  the  optic  nerves  have  their 
final  endings.  If  these  lobes  are  destroyed,  psychical  blind- 
ness results;  if  they  are  stimulated,  we  have  the  sensa- 
tion of  sight,  no  matter  how  this  stimulation  is  brought 
about.  This  led  someone  to  say  that  if  the  auditory  or 
eighth  cranial  nerve  which  leads  from  the  ear  to  that 
portion  of  the  brain  where  auditory  sensations  originate, 
could  be  made  to  conduct  impulses  to  the  occipital  lobes,  we 
would  be  able  to  see  the  music  played  by  a  band. 

Seeing  that  we  are  not  acquainted  with  the  nature  of  the 
change  set  up  in  the  rods  and  cones  and  the  impulse  trans- 
mitted by  the  optic  nerve,  we  shall  leave  this  subject  and 
next  discuss  the  impressions  that  we  receive  when  light 
falls  into  our  eye.  The  retino-cerebral  mechanism  gives  rise 
to  three  sensations :  light,  color,  and  space  sensations.  Cer- 
tain animals  possess  organs  which  give  them  impressions  of 


174  '^'^^    PHYSIOLOGY    OF    VISION. 

light  only.  A  differentiation  of  nerve  fibers  has  taken  place 
of  such  a  nature  that  they  are  affected  by  light ;  but,  as  no 
lens  is  present,  there  are  no  images  cast  upon  the  nerve 
fibers,  and  all  that  these  animals  perceive  is  light.  They  can 
distinguish  between  light  and  darkness,  but  they  have  no 
idea  of  form  or  distance  and  most  likely  not  of  color. 

In  our  discussion  of  the  sensation  of  light,  we  may  first 
inquire  into  the  relation  existing  between  the  stimulus  (the 
objective  light)  and  the  resulting  sensation.  How  long 
must  a  light  act  and  with  what  intensity  must  light  act  in 
order  to  be  seen?  It  has  been  found  that  we  can  perceive 
a  light  lasting  i-8,ooo,oooth  of  a  second.  We  may,  there- 
fore, state  that,  so  far  as  we  know  at  present,  if  the  light 
has  a  sufficient  intensity,  no  matter  how  short  its  duration, 
it  is  visible.  It  must  have  a  certain  intensity  for  reasons 
which  we  will  take  up  in  our  next  lecture. 


LECTURE  V. 

At  the  close  of  the  last  lecture  we  learned  that  the  length 
of  time  required  for  the  light  to  act  upon  the  retina  in  order 
that  it  can  be  perceived  is  extremely  short,  a  spark  of  light 
from  a  revolving  mirror  lasting  only  i -8,ooo,oooth  of  a 
second  can  be  perceived.  However,  the  sensation  produced 
is  not  as  short  as  this.  The  curved  line  in  Fig.  31  repre- 
sents the  intensity  of  the  visual  sensation ;  the  higher  a  cer- 
tain part  of  the  curve  is  above  the  base  line  (nm),  the 
greater  the  intensity  of  the  sensation  at  that  moment.  Let 
us  suppose  that  the  light  which  produced  this  sensation 


Fig.  31. 


lasted  from  A  to  C.  The  curve  of  sensation  can  be  readily 
divided  into  four  parts :  A  B,  B  C,  C  D,  and  D  E.  At  A 
the  light  begins  to  act  upon  the  retina  and  the  sensation 
begins;  however,  it  will  be  noticed  that  the  sensation  does 
not  reach  its  maximum  immediately,  but  gradually  increases 
in  intensity  until  at  B  it  attains  its  greatest  intensity. 

From  this  it  is  evident  that  a  dim  light  acting  for  a 
longer  length  of  time  may  produce  a  stronger  sensation 
than  a  bright  light  acting  for  a  very  short  length  of  time. 
Suppose  an  electric  spark  lasting  but  i-i, 000,000th  part  of 
a  second  falls  upon  the  eye.     Tn  that  length  of  time  the 

ns 


176  THE    PHYSIOLOGY    OF    VISION. 

sensation  has  reached,  let  us  say,  only  one-fourth  the  value 
that  it  would  reach  if  the  spark  should  last  for  one  whole 
second.  Let  us  now  suppose  that  a  second  light  having 
only  one-half  the  brilliancy  of  the  electric  spark  but  lasting 
one  second  falls  upon  the  eye.  In  that  length  of  time  the 
sensation  has  reached  its  full  value  which  is,  therefore, 
twice  as  great  as  that  caused  by  the  electric  spark.  You  see 
that  the  eye  is  similar  to  a  photographic  camera  in  that  the 
efifect  increases  with  the  length  of  exposure;  there  is  an 
accumulative  effect. 

This  naturally  leads  us  to  ask,  how  much  light  must 
be  thrown  into  the  eye  in  order  to  produce  a  sensation? 
You  will  remember  that  irritability  was  defined  as  the 
power  of  a  living  being  to  respond  to  a  stimulus.  The 
smallest  amount  of  a  stimulus  that  can  produce  a  sensation 
is  called  its  liminal  intensity.  What  is  the  liminal  intensity 
of  the  light  entering  the  eye?  This  is  very  difficult  to 
state,  for  it  is  not  an  easy  matter  to  measure  and  graduate 
the  amount  of  light.  For  certain  sense  organs,  such  as 
touch,  it  is  less  difficult  to  determine  the  liminal  intensity 
of  the  stimulus  because  one  can  readily  measure  the  amount 
of  pressure  applied  in  milligrams  or  fractions  of  a  milli- 
gram. However,  attempts  have  been  made  to  determine  how 
great  the  luminosity  must  be  in  order  to  produce  a  sensa 
tion,  and  it  was  found  that  a  sheet  of  white  paper  illumin- 
ated bv  a  standard  candle  can  still  be  perceived  at  a  dis- 
tance of  200  or  250  meters.  If  the  distance  is  greater  than 
this,  it  can  no  longer  be  perceived. 

Moreover,  the  liminal  intensity  depends  to  a  large  extent 
upon  the  condition  of  the  eye.  It  is  a  well-known  fact  that 
if  your  eye  is  accustomed  to  bright  light,  you  do  not  readily 
perceive  a  very  dim  light,  but  after  remaining  in  a  dark 
room  for  some  time,  the  dim  light  looks  quite  bright.  In 
determining  the  liminal  intensity  of  light,  the  retina  should 
be  in  resting  condition,  in  other  words,  it  should  be  as 
sensitive  as  possible.    Remaining  in  the  dark  for  three  min- 


THE    PHYSIOLOGY    OK    VISION.  I77 

utes  increases  the  irritability  of  the  retina  from  ten  to 
fifteen  times,  while  after  a  stay  of  two  hours  the  irritability 
is  thirty-five  times  as  great  as  when  the  eye  is  illuminated 
by  daylight. 

But  even  under  these  circumstances  all  eyes  are  not  the 
same ;  sailors  see  land  at  a  distance  when  a  landsman 
cannot  see  anything.  Artists  and  orientals  have  a  much 
higher  developed  sense  of  color  and  light  than  other  people. 
So  the  threshold  of  irritability  or  liminal  intensity  varies  in 
different  people  and  depends  upon  the  condition  of  the  eye 
and  upon  previous  training.  Neither  is  the  threshold  of  irri- 
tability the  same  for  all  portions  of  the  retina.  In  the  last 
lecture  we  learned  that  when  we  wish  to  see  an  object  dis- 
tinctly, the  focus  of  that  object  always  falls  on  the  fovea 
centralis,  or  yellow  spot.  From  this  you  might  infer  that 
this  portion  of  the  retina  is  also  the  most  sensitive  to  light, 
but  the  following  experiment  indicates  that  this  is  not  true 
for  light  of  all  intensities. 

Experiment  i6.  Turn  the  gas  jet  very  low  so  that  a  mere 
spark  of  light  remains  and  view  this  from  a  distance  of  about 
ten  or  fifteen  feet.  Close  one  eye  and  look  directly  at  the 
gas  jet;  notice  its  luminosity.  Now  look  a  few  inches  to 
one  side  of  the  light,  seeing  this  by  indirect  vision,  that  is, 
letting  the  focus  fall  outside  of  the  yellow  spot;  the  light 
appears  very  much  brighter  than  it  did  before. 

This  experiment  demonstrates  that  while  in  the  ordinary 
sense  the  fovea  centralis  has  the  greatest  visual  acuity,  the 
threshold  of  irritability  is  less  for  those  portions  of  the 
retina  immediately  surrounding  the  fovea  centralis  than  for 
the  fovea  itself.  This  has  received  the  following  explana- 
tion. As  was  stated  in  Lecture  IV,  the  fovea  centralis  is 
well  supplied  with  cones,  but  the  rods  (see  Fig.  27)  are 
lacking  here.  In  the  peripheral  portions  of  the  retina,  that 
is,  in  the  retina  outside  of  the  fovea,  we  find  both  rods  and 
cones,  the  number  of  cones  diminishing  rapidly  as  we  pro- 
ceed toward  the  limits  of  the  retina.     Some  physiologists 


lyS  THE    PHYSIOLOGY    OF     VISION. 

suppose  that  the  rods  and  cones  do  not  perform  the  same 
functions,  and  that  the  rods  are  color  bhiid  but  are  espe- 
cially adapted  for  viewing  light  (not  color)  of  low  in- 
tensity. Some  hold  that  this  function  of  the  rods  is  due 
to  the  rhodopsin  or  visual  purple,  which,  as  we  stated  in 
the  previous  lecture,  is  only  found  in  the  rods. 

A  subject  closely  related  to  this  is  known  as  the  law  of 
Weber,  which  states  by  how  much  a  light  must  be  in- 
creased in  order  that  we  maybe  able  to  perceive  the  increase. 
As  this  law  holds  good  within  certain  limits  for  other  sen- 
sations, we  may  for  a  few  minutes  leave  the  subject  of 
light.  Suppose  I  draw  a  line  of  a  certain  length ;  how  much 
longer  must  I  draw  a  second  line  in  order  that  the  two 
may  be  distinguished?  It  has  been  found  that  if  the  first 
line  is  lOO  mm.  long,  the  length  of  the  second  line  must 
be  at  least  105  mm.  Again,  if  the  first  line  is  200  mm.,  the 
second  line  must  be  210  mm.;  if  the  first  line  is  1,000  mm., 
the  second  line  must  be  1,050  mm.  The  difference  between 
100  and  105  is  five,  and  the  ratio  of  this  difference  to  the 
length  of  the  first  line  is  5-100,  or  1-20.  The  difference 
between  1,000  and  1,050  is  fifty,  and  here  the  ratio  of  the 
difference  to  1,000  is  50-1,000,  or  again  1-20.  Hence  we 
may  state  this  in  the  following  general  manner:  In  order 
that  a  difference  in  the  length  of  two  lines  shall  be  per- 
ceived, the  difference  between  the  two  lines  must  be  a  certain 
fraction  (ratio)  of  the  shortest  line;  this  fraction  is  con- 
stant no  matter  what  the  length  of  the  lines  may  be  and  its 
value  is  1-20. 

To  a  certain  extent  this  also  holds  true  for  pressure  sen- 
sations, as  the  following  shows.  If  your  hands  rests  on  the 
table,  and  if  a  weight  is  placed  on  the  hand,  you  experi- 
ence a  sensation  of  a  certain  strength.  How  large  a  weight 
must  be  added  in  order  that  you  can  perceive  a  change  in 
the  intensity  of  the  sensation?  It  has  been  found  that  if 
the  original  weight  is  ten  grams,  one  gram  must  be  added 
to  it  in  order  that  a  change  in  the  sensation  can  be  per- 


THE    PHYSIOLOGY    OF    VISION.  I79 

ceived.  Instead  of  ten  grams,  the  weight  might  have  been 
ten  grains,  ten  ounces,  or  any  other  weight ;  in  each  case 
i-io  of  the  original  weight  must  be  added  to  produce 
a  change  in  the  sensation.  If  less  than  i-io  is  added,  you 
are  unable  to  tell  the  difference. f  We  may  generalize  this 
as  follows :  Whatever  the  strength  of  the  stimulus  may  be, 
it  must  be  increased  by  the  same  fraction  in  order  that  a 
difference  in  the  sensation  may  be  perceived.  It  will  be 
noticed  that  this  is  the  same  conclusion  that  we  arrived  at 
in  discussing  the  least  perceptible  difference  in  the  lengths 
of  two  hues. 

Does  our  sensation  of  sight  follow  the  same  law?  It 
does.  Suppose  this  room  is  lit  by  one  hundred  candles,  how 
many  candles  must  be  added  in  order  that  we  could  per- 
ceive an  increase  in  the  luminosity?  One  candle  must  be 
added.  That  is,  we  can  tell  the  difference  between  the  light 
from  one  hundred  and  that  from  one  hundred  and  one 
candles.  The  ratio  of  the  difference  to  the  original  stimulus 
is  I -100,  hence  i-ioo  is  the  ratio  by  which  the  light  must  be 
decreased  or  increased  in  order  to  cause  a  difference  in  the 
sensation.  If  instead  of  one  hundred  there  are  ten  candles, 
then  i-ioo  of  lo  or  i-io  of  a  candle  must  be  added. 
Whether  the  light  is  furnished  by  candles,  lamps,  or  electric 
lights  does  not  alter  the  rule. 

We  may  once  more  state  this  in  general  terms:  The 
smallest  change  in  the  magnitude  of  the  stimulus  (light, 
pressure,  etc.)  which  we  can  appreciate  through  a  change  in 
our  sensation  is  always  the  same  ratio  of  the  total  stimulus. 
This  is  called  Weber's  law. 

Weber's  law  explains  a  great  many  phenomena  that  are 
familiar  to  you.  We  have  time  to  call  attention  to  but  a 
few.  A  candle  is  burning  in  a  dark  room,  and  casts  a 
shadow  of  an  object  on  the  floor.  If  a  little  daylight  is  let 
into  the   room,  the  shadow   begins   to   fade   away,   and   if 


tThis  ratio  differs  for  different  regions  of  the  body. 


I  So 


THE    PHYSIOLOGY    OF'    VISION. 


the  bright  sunhght  falls  upon  the  floor,  the  shadow  cast  by 
the  candle  disappears.  The  reason  for  this  is  as  follows.  In 
Fig.  32  let  O  be  the  object  whose  shadow  falls  upon  the 
floor  at  P.  This  portion  of  the  floor,  therefore,  receives 
no  light,  while  the  remainder  of  the  floor,  Q  and  Q,  receives 
the  full  light  of  the  candle,  the  intensity  of  which  we  shall 
call  one.    Now  the  full  sunlight  is  let  in  which  lights  up  the 


^*?. 


Fig.  32, 


whole  floor,  the  spot  P  as  well  as  the  neighboring  portions, 
Q  and  Q.  Let  us  call  the  intensity  of  the  sunlight  1,000, 
that  is,  we  shall  assume  that  it  is  one  thousand  times  as 
bright  as  the  candle  light.  The  portions  of  the  floor  Q  and 
Q  receive  light  both  from  the  candle  and  from  the  sun, 
hence,  the  total  luminosity  of  Q  is  1,001.  The  place  P 
receives  light  only  from  the  sun,  hence,  its  luminosity  is 
only  1,000.  The  difference  between  the  luminosity  of  P 
and  O  is  therefore  i,  which  is  i-iooo  of  the  total  lumin- 
osity of  P.  We  have  seen  that  two  lights  must  differ  by 
i-iooth  part  of  the  weakest  light  in  order  that  we  can  per- 
ceive the  difference.  It  is,  therefore,  obvious  that  the  dif- 
ference between  the  luminosity  of  P  and  Q  cannot  be  pei- 
ceived ;  the  floor  will  have  a  uniform  appearance. 


THE    PHYSIOLOGY    OF    VISION.  l8l 

We  read  a  printed  page  by  the  light  of  a  good  lamp 
as  well  as  in  daylight,  although  the  daylight  is  far  more  in- 
tense than  the  lamp-light.  This  also  is  according  to  Web- 
er's law.  Let  us  suppose  that  the  intensity  of  the  lamp- 
light is  10,  that  of  daylight  i,ooo.  In  the  lamp-light  the 
amount  of  light  that  you  receive  from  the  ',vhite  paper  is 
ten  (not  taking  into  consideration  the  absorption  of  light) 
and  the  amount  of  light  received  from  the  black  letters  is, 
let  us  say,  one.  The  ratio  of  the  difference  (9)  to  the  total 
illumination  (10)  is  9-10;  as  this  difference  is  greater  than 
one-hundredth,  you  can  read  the  letters.  When  you  read 
in  daylight,  the  amount  of  light  from  the  white  paper  is 
one  hundred  times  greater,  that  is,  it  is  one  thousand,  but 
the  light  from  the  letters  has  also  been  increased  by  one 
hundred,  that  is,  it  is  equal  to  100.  The  ratio  of  the  dif- 
ference to  the  total  illumination  is  900-iouo  or  9-10,  the 
same  as  in  lamp-light.  Hence,  you  can  see  to  read  by  a 
good  lamp  just  as  well  as  in  perfect  daylight. 

Why  do  we  not  see  the  stars  in  the  day  time?  Because 
the  amount  of  light  which  they  send  is  so  small  compared 
with  the  amount  of  sunlight,  that  the  difference  falls  below 
the  fraction  of  i-ioo.  If  the  light  received  from  a  star 
were  i-ioo  part  of  the  light  received  from  the  sun,  that 
star  would  be  visible  by  day ;  but  as  no  star  sends  this 
amount  of  light,  we  are  able  to  see  them  only  when  the 
sunlight  is  decreased. 

Weber's  law  holds  good,  however,  only  for  certain  in- 
tensities of  light ;  in  very  feeble  or  very  bright  light  other 
factors  enter.  Suppose  you  are  reading  by  lamp-light  and 
that  the  amount  of  light  is  gradually  decreased,  at  first 
reading  is  still  possible,  for  the  reasons  given  above,  but  as 
the  light  becomes  less  and  less,  it  becomes  more  and  more 
difficult  to  distinguish  the  print,  until  finally  it  is  impossible. 
The  reason  for  this  is  that  the  intrinsic  light  of  the  eye  (see 
Lecture  IV)  is  added  to  both  the  light  from  the  paper 
and  that  from  the  printed  letter,  raising  the  intensity  of  both 


1 82  THE    PHYSIOLOGY    OF    VISION. 

SO  that  the  difference  is  imperceptible.  On  the  other  hand, 
it  is  generally  impossible  to  see  sun  spots  with  the  naked 
eye,  while  these  are  visible  if  the  eye  is  protected  by  smoked 
glass.  This  is  not  because  the  difference  of  luminosity  be- 
tween the  sunspot  and  the  surrounding  surface  of  the  sun 
is  not  sufficiently  great  for  us  to  perceive  it,  but  because  the 
glare  of  the  sun  blinds  the  eye,  which  then  no  longer  fol- 
lows Weber's  law. 

In  discussing  the  liminal  intensity  of  light  we  spoke  only 
of  wdiite  light,  but  there  is  also  a  liminal  intensity  of  colored 
light.  In  determining  this  it  was  found  that  if  a  very 
feeble  light  is  cast  upon  a  piece  of  colored  paper,  the  light 
is  colorless  (gray).  If  the  amount  of  light  is  increased,  the 
proper  color  is  perceived.  Consequently  in  colors  we  have 
two  thresholds ;  first,  the  absolute  or  light  threshold,  and, 
second,  the  chromatic  or  color  threshold.  All  colors  look 
gray  in  very  dim  light,  for  which  the  following  reason  has 
been  ascribed.  The  small  amount  of  light  from  a  colored 
object  has  sufficient  intensity  to  stimulate  the  rods,  which, 
as  we  have  seen  a  moment  ago,  have  a  lower  threshold  of 
irritability,  but  have  no  color  vision ;  this  intensity  is  not 
sufficient,  however,  to  stimulate  the  cones,  whose  stimula- 
tion gives  rise  to  color  sensation.  Hence  the  colored  light 
of  low  intensity  appears  colorless. 

When  white  light  or  mixed  light  falls  upon  the  retina, 
the  maximum  effect  of  all  the  colors  is  not  perceived  at  the 
same  time.  At  the  beginning  of  this  lecture,  we  stated  that 
the  sensation  does  not  reach  its  maximum  immediately,  but 
that  the  sensafon  increases  gradually  (see  Fig.  31).  Now 
the  sensation  of  red  light  reaches  its  maximum  sooner 
than  that  of  green  light;  this  gives  rise  to  a  curious  color 
phenomenon,  which  can  be  demonstrated  by  means  of  the 
Benham  spectrum  top  (Fig.  33).  This  consists  of  a  disk, 
half  white  and  half  black.  On  the  white  surface  are  placed 
strips  of  black  paper  in  concentric  circles.  If  this  disk  is 
rotated  on  a  wheel  in  the  direction  indicated  by  the  arrow 


THE    PHYSIOLOGY    OF    VISION.  183 

(Fig.  ^;^)  the  concentric  bands  give  rise  to  color  sensa- 
tions which  are  from  within  outward,  red,  brown,  oUve, 
green,  blue.  If  the  wheel  is  turned  in  the  opposite  direc- 
tion, the  position  of  the  colors  is  reversed. 

A  phenomenon  somewhat  similar  to  this  is  the  "flicker" 
phenomenon.  On  a  wheel  rotate  a  disk  half  of  which 
is  white  and  the  other  half  black.  Cast  a  strong  light  upon 
it.  If  this  is  rotated  with  sufficient  speed,  a  uniform  gray 
is  produced,  which  will  be   discussed  in   the  next  lecture. 


Fi^.  33.    Disk  of  nenhatn. 

But  if  the  speed  is  decreased  a  flickering  is  produced,  and  a 
pattern  of  colors,  red,  blue,  green,  yellow,  etc.,  is  seen. 
By  increasing  or  decreasing  the  speed,  this  pattern  changes 
in  a  most  surprising  manner.  No  adequate  explanation  has 
yet  been  given  for  this  curious  phenomenon.  It  is  not  due 
to  the  decomposition  or  analysis  of  the  white  light,  for  it 
can  also  be  observed  in  monochromatic  light,  that  is,  in  light 
of  one  color. 

Charpentier's  Bands.  We  shall  now  rotate  on  the  wheel 
a  disk  having  three  quadrants  black  and  the  remaining 
quadrant  white.  On  rotating  this  slowly  and  keeping  your 
line  of  vision  fixed  upon  the  center,  you  will  notice  a  narrow 


184  THE    PHYSIOLOGY    OF    VISION 

grayish  sector,  like  a  shadow,  situated  in  the  white  quad- 
rant, at  the  point  A,  Fig.  34.  By  looking  attentively,  it  may 
be  possible  to  locate  a  second  band  in  the  white  sector  (at 
B,  Fig.  34).  These  bands  are  known  as  Charpentier's 
bands.  It  would  lead  us  too  far  into  the  subject  to  attempt 
an  explanation. 

The  last  subject  with  which  I  shall  trouble  you  in  this 
lecture  is  irradiation. 


Fig.  84. 

Experiment  17.  Cut  out  two  small  squares  of  paper  of 
exactly  the  same  size,  one  of  black  and  the  other  of  white 
paper.  Place  the  white  square  on  a  black  background  (on 
black  cloth  or  paper)  and  the  black  square  on  a  white 
background.  The  white  square  on  the  black  background 
appears  considerably  larger  than  the  black  square  on  the 
white  background.  This  is  supposed  to  be  due  to  a  spread- 
ing of  the  sensation  of  white  over  the  black,  thereby  enlarg- 
ing the  white  square ;  but  it  is  possible  that  spherical  aber- 
ration also  plays  a  part.  The  following  experiment  is  also 
an  illustration  of  irradiation. 

Experiment  18.  Hold  the  edge  of  a  knife  or  card  hori- 
zontally so  that  it  shuts  off  half  of  the  flame  of  a  gas  jet. 
Use  a  small  flame.  View  the  flame  just  over  the  edge  of  the 
card,  and  it  appears  as  if  the  opaque  object  is  notched  at 


THE    PHYSIOLOGY    OF    VISION.  185 

the  point  where  the  object  and  the  flame  meet. 

Two  more  ilktstrations  of  irradiation  are  the  well-known 
facts,   that  people   look   smaller    in    dark    than    in    white 
clothes,  and  that  the  darker  old  moon  in  the  arms  of  the  new 
appears   smaller  than   the   lighter   strip   of  new  moon   en 
circling  it. 


LECTURE  VI. 
We  have  learned  in  the  last  lecture  that  the  sensation 
produced  by  the  stimulation  of  the  retina  does  not  reach 
its  maximum  immediately,  but  gradually  increases  in 
strength,  till,  after  a  certain  length  of  time,  it  reaches  its 
greatest  intensity.  This  is  graphically  represented  by  the 
curve  in  Fig.  35.  Suppose  the  stimulation  by  light  lasted 
from  A  to  C,  the  curve  of  sensation  gradually  increases 
from  A  to  B. 


Fig.  35. 

In  this  lecture  we  wish  to  consider  the  third  part  of  the 
curve,  that  from  C  to  D.  As  before  stated,  let  us  suppose 
that  the  stimulation  ceases  at  C ;  it  will  be  noticed  that  the 
curve  of  sensation  does  not  immediately  drop  down  to  the 
base  line.  The  sensation  continues  for  a  certain  length  of 
time,  from  C  to  D,  gradually  fading  away ;  in  other  words, 
the  sensation  lasts  longer  than  stimulation.  If  the  stimula- 
tion (light)  lasts  for  one  second,  the  sensation  produced 
by  this  stimulation  lasts  one  second  plus  a  fraction  of  a 
second. 

We  may,  therefore,  say  there  is  an  after  eflfect,  an  after 
sensation.  And  this  is  true  not  only  of  our  organs  of  sight, 
but  also  of  other  sense-organs.  If  a  weight  is  placed  on 
your  hand,  a  sensation  of  pressure  is  experienced ;    if  the 

186 


THE    PHYSIOLOGY    OF    VISION.  187 

weight  is  removed,  the  sensation  remains  for  a  brief  length 
of  time. 

The  fact  that  the  sensation  lasts  longer  than  the  stimu- 
lation  leads  to  the   fusion  of  sensations.     Since  we  were 


Fig.  36.      Whirling  machine  for  mixing  colors. 


boys,  we  were  familiar  with  the  fact  that  if  a  live  coal  is 
rapidly  swung  around,  a  circle  of  light  is  seen,  especially 
if  this  is  done  in  the  dark.  A  similar  phenomenon  can 
be  demonstrated  on  the  color  wheel  (Figs.  36  and  36a).* 


*Nearly  all  the  experiments  related  in  this  and  in  the  following 
lecture  were  performed  on  the  color  wheel,  an  instrument  by  means 
of  which  disks  of  various  colors  can  be  rapidly  rotated.  Those  de- 
siring to  follow  these  experiments  at  home,  can  procure  from  the 
Milton  Bradley  Co.,  Springfield,  Mass.,  a  simple  "Color  Top"  with  the 
necessary  disks,  which  answers  the  purpose  admirably.  Not  only  can 
this  "top"  be  used  in  making  the  experiments  outlined  in  these  lec- 
tures, but  it  will  furnish  very  wholesome  amusement  to  the  children. 
The   cost  of  top   and  disks   is   six  cents,  postpaid. 


l88  THE    PHYSIOLOGY    OF    VISION. 

Experiment  19.  On  the  spindle  of  the  color  top  arrange 
a  white  and  black  disk,  so  that  one-half  of  the  field  is  white 
and  the  other  half  black.  On  rotating  these  disks  slowly, 
you  see  the  white  and  black  sectors  alternately  and  sep- 
arately; but  as  the  speed  increases  you  notice  that  fusion 
takes  place  until  finally  there  is  no  white  or  black,  but  a 
perfectly  uniform  gray. 

This  fusion  of  the  two  stimuli,  white  and  black,  depends 
upon  the  fact  that  the  sensation  lasts  longer  than  the  stimu- 
lation. Suppose  Fig.  37  represents  the  retina  and  that  the 
image  of  the  white  sector  falls  at  A  and  that  of  the  black 
sector  at  B.  If  the  wheel  is  rotated  slowly,  the  position  of 
the  image  of  the  white  area  changes  from  A  to  B,  and  the 


Fig.  36A.     Color  Discs 

sensation  at  A  disappears  while  a  new  sensation  of  the 
white  area  is  formed  at  B.  But  rotate  the  wheel  faster. 
The  image  of  the  white  area  leaves  A,  the  stimulation  of  the 
white  light  ceases,  but  the  sensation  lasts,  let  us  say  1-50 
second  longer.  If  the  wheel  is  rotated  so  fast  that  the 
image  of  the  white  area  is  at  A  again  before  the  expiration 
of  the  1-50  second,  then  the  second  sensation  fuses  with  the 
first  sensation  which  remained,  and  consequently  you  are 
not  conscious  of  that  fact  that  the  white  area  has  changed 
its  position;  a  uniform  sensation  is  produced.  This  sensa- 
tion is  gray,  because  the  black  sector  of  the  disk  reduces  the 
sensation  of  luminosity  produced  by  the  white  disk. 

This  after-image,  which  outlasts  the  stimulation  and  upon 
which  the  fusion  of  the  sensations  depend,  is  called  the 
positive  after-image.    How  long  is  the  duration  of  the  posi- 


THE    PHYSIOLOGY    OF    VISION.  189 

tive  after-image?  This  can  be  determined  by  ascertaining 
how  often  we  must  rotate  the  wheel  in  the  above  experi- 
ment in  order  to  obtain  complete  fusion.  We  shall  throw 
a  dim  light  upon  the  wheel  and  rotate  with  sufficient  rapid- 
ity to  cause  fusion.  Now  we  will  increase  the  light;  you 
notice  that  I  must  rotate  the  wheel  faster  in  order  to  cause 
fusion.  This  indicates  that  the  number  of  stimulations  per 
second  must  be  very  much  greater  in  bright  light  than  in 
dim  light ;  the  positive  after-image  lasts  longer  in  dim  light 
than  in  bright  light.  In  dim  light  about  thirteen  stimula- 
tions per  second  are  necessary ;  in  the  bright  sun  light  sixty 
or  more  are  needed. 


Fig.  37. 

There  is  another  form  of  positive  after-image  which  is 
extremely  interesting,  which  you  can  demonstrate  for  your- 
self, by  the  following  experiment.  As  this  experiment  can 
be  made  only  when  the  eye  is  in  a  perfectly  fresh  condition, 
it  is  best  to  make  it  immediately  after  waking.  When  you 
awake  in  the  morning  look  out  of  the  window  for  a  short 
length  of  time,  about  five  or  six  seconds ;  notice  the  trees 
and  houses.  Cover  your  face  with  the  bed  clothes  and  you 
will  see  a  reproduction  of  the  houses  and  trees.  This  after- 
image is  so  astonishingly  perfect  and  clear  that  one  can't 
help  miagining  that  the  eyes  are  open  and  still  looking  at 
the  objects.    This  is  also  a  positive  after-image,  but  differs 


190  THE    PHYSIOI.OGY    OF    VISION. 

from  the  ordinary  positive  after-image  in  its  long  dura- 
tion. In  the  above  experiment  it  is  not  sufficient  to  close 
the  eyes,  for  a  certain  amount  of  light  penetrates  the 
eyelids. 

You  are  all  familiar  with  the  positive  after-image  ob- 
tained by  looking  at  the  sun.  If  you  look  at  the  red  setting 
sun,  a  very  lasting  after-image  is  seen  which  generally 
appears  as  a  light  luminous  spot  when  the  eyes  are  closed, 
while  it  appears  as  a  dark  circle  when  you  look  at  a  bright 
surface,  as  the  sky.  If  you  look  steadily  for  some  time 
at  the  setting  sun,  the  after-image  will  be  seen  in  colors 
which  gradually  change.  At  first  the  after-image  is  bluish 
green ;  this  gives  place  to  a  green,  and  this  in  turn  to  a  blue ; 
then  violet,  pink,  orange,  and  green  are  seen  successively. 
What  this  change  in  color  is  due  to,  is  not  well  under- 
stood. In  some  cases  positive  after-images  have  been  known 
to  exist  for  a  life  time;  no  doubt  in  these  instances  there 
is  an  actual  injury  to  the  retina  caused  by  too  strong  stimu- 
lation. 

The  retina  of  our  eye  is  stimulated  by  light,  and  con- 
sciousness is  affected ;  but  light  is  not  the  only  agency  that 
can  thus  stimulate  the  optic  nerve.  For  instance,  some  peo- 
ple see  stars,  although  there  may  be  no  light  present. 
Mechanical  stimulation  of  the  optic  nerve,  as  by  cutting 
the  nerve  in  operations  for  the  removal  of  the  eyeball, 
also  produces  a  sensation  of  light.  Passing  an  electric  cur- 
rent through  the  eye  also  calls  forth  a  sensation  of  light. 
Again,  when  the  ear  or  the  auditory  nerve  is  stimulated, 
whether  by  sound  waves,  by  a  mechanical  blow,  or  by  an 
electric  current,  the  result  is  always  a  sensation  of  sound. 
These  facts  have  led  physiologists  to  the  theory  of  specific 
energy  of  nerves,  by  which  we  mean  that  the  nature  of  the 
sensation  (whether  sound,  sight,  taste,  etc.)  is  not  deter- 
mined by  the  nature  of  the  stimulation  applied  to  the  par- 
ticular nerve,  but  by  the  endings  of  that  nerve  in  the  brain. 
This  is  very  well  illustrated  in  people  whose  feet  have  been 


THE    PHYSIOLOGY    OF    VISION.  191 

amputated  and  who  have  subsequently  complained  of  their 
painful  corns.  The  nerve  that  originally  supplied  the  toes  is 
stimulated  by  the  contracting  healing  tissue,  and  the  result 
in  consciousness  is  the  same  as  if  the  corn  was  irritated. 

Although  the  eye  can  be  stimulated  in  these  various  ways, 
yet  it  is  evident  that  the  eye  is  not  built  for  the  reception 
of  such  stimuli.  In  fact,  the  organ  of  vision  is  constructed 
in  such  a  manner  that  it  can  be  best  stimulated  by  light,  i.  e., 
ether  vibrations ;  hence  this  form  of  st'mulus  is  called  the 
adequate  stimulus.  Other  sense  organs  also  have  their  ade- 
quate stimuli,  that  for  the  ear  being  the  vibrations  of  air,  or 
sound  waves. 

As  ether  vibrations  are  the  adequate  stimulus  for  the  eye, 
it  is  necessary  that  we  study  them  a  little  more  closely.  The 
lowest  ether  vibrations  of  which  we  have  definite  knowledge 
have  107,000,000,000,000  vibrations  per  second,  and  the 
shortest  waves  vibrate  40,000,000,000,000,000  times  per  sec- 
ond. However,  all  these  vibrations  of  ether  do  not  affect 
the  eye.  Our  eye  is  stimulated  only  by  vibrations  ranging 
from  392,000,000,000,000  to  757,000,000,000,000  vibrations 
per  second.  If  the  number  of  vibrations  is  less  than  390,- 
000,000,000,000  or  greater  than  760,000,000,000,000  per 
second,  they  do  not  affect  the  eye,  hence  produce  no  sensa- 
tion of  vision  (light).  But  these  vibrations  manifest  them- 
selves to  us  in  other  ways.  If  ether  vibrations  of,  say  300,- 
000,000,000,000  fall  upon  the  skin,  certain  nerves  are  stimu- 
lated and  we  have  a  sensation  of  heat.  The  shorter  wave 
lengths,  i.  e.,  waves  having  greater  number  of  vibrations 
per  second,  have  the  power  of  changing  certain  pigments, 
as  is  seen  in  the  bleaching  of  colors  of  cloth  or  paper ;  for 
this  reason  these  short  waves  are  sometimes  called  actinic, 
or  chemical,  rays.  These  short  waves  do  not  affect  the 
human  eye,  but  it  is  possible  that  other  animals  can  perceive 
these  vibrations.  This  is  rendered  very  likely  from  what 
we  know  of  other  sense  organs,  such  as  the  ear.  Our  car 
can  perceive  sounds  ranging  from  about  16  to  40,000  vibra- 


192  THE    PHYSIOLOGY    OF    VISION. 

tions  per  second.  If  a  sound  of  more  than  40,000  vibrations 
per  second  strikes  the  human  ear,  it  causes  no  sound  sensa- 
tion, but  a  dog  will  respond  to  these  sound  waves.  Lub- 
bock states  that  ants  and  certain  water  beetles  (Daphnia) 
seem  to  be  able  to  see  ultra-violet  rays. 

The  rays  of  the  sun  which  are  visible  to  us  have  different 
rates  of  vibration,  and  according  to  the  rate  of  vibration 
affect  our  eye  differently.  If  the  rate  of  vibration  is  395,- 
000,000,000,000  per  second,  the  sensation  called  forth  is  red ; 
if  the  rate  is  740,000,000,000,000,  we  experience  a  sensation 
of  violet.  By  means  of  a  prism  the  white  sunlight  can  be 
decomposed,  i.  e.,  the  rays  of  various  lengths  can  be  sep- 
arated from  each  other ;  the  result  is  called  the  spectrum  of 
the  sunlight.  The  following  table  gives  the  number  of  vi- 
brations and  the  wave  length  of  the  seven  main  colors  seen 
in  the  solar  spectrum : 

Number  of  vibrations  Wave  lengths  in 

per  second.  niilUmeters.* 

Red    395,000,000,000,000  0.0007604 

Orange    503,000,000,000,000  0.0005972 

Yellow    517,000,000,000,000  0.0005808 

Green 570,000,000,000,000  0.0005271 

Cyan-blue    606,000,000,000,000  0.0004960 

Indigo    635,000,000,000,000  0.0004732 

Violet    740,000,000,000,000  0.0004059 

In  this  table  are  noted  only  the  seven  chief  colors  which 
most  people  recognize  in  the  rainbow.  However,  if  you 
observe  the  spectrum  carefully,  you  will  notice  that  these 
seven  colors  gradually  shade  into  each  other,  that  there  is  no 
sharp  line  of  demarkation  between  them,  and  that  many 
more  colors  can  be  seen.  Some  state  that  at  least  160  colors 
can  be  recognized.  And  if  these  colors  are  mixed  in  vari- 
ous ways,  especially  if  they  are  mixed  with  white  and  with 
black,  it  is  estimated  by  V.  Kries  that  we  can  appreciate 
500,000  different  colors,  tints,  and  shades. 

*A  millimeter  Is  equal  to  about  1-25  (0.0393704)  of  an  inch. 


THE    PHYSIOLOGY    OF    VISION.  I93 

Colors  have  three  distinguishing  marks.  First,  we  have 
the  hue,  or  tone,  of  the  color.  By  the  tone  of  the  color  we 
mean  what  is  ordinarily  called  the  color,  e.  g.,  red,  blue, 
green.  Again,  we  can  speak  of  the  purity,  or  saturation,  of 
a  color  aside  from  its  tone,  by  which  we  mean  the  amount 
of  white  light  there  iS  mixed  with  the  pure  color. 

Experiment  20.  Take  the  red  disk  which  is  found  in  the 
outfit  of  the  color  top.  Let  us  say  that  this  is  a  perfectly 
saturated  color,  that  is,  its  purity  is  one  hundred  per  cent. 
On  the  color  top  combine  75  per  cent  red  and  25  per  cent 
white,  and  rotate  fast  enough  to  cause  complete  fusion. 
Although  the  resulting  color  is  still  red,  the  purity,  or  sat- 
uration, is  decreased ;  the  color  is  a  light  or  pale  red. 

The  more  white  light  is  mixed  with  the  red,  the  less  the 
saturation  and  the  paler  the  color.  Tints  are  produced  by 
mixing  a  saturated  color  with  white  light;  pink,  for  ex- 
ample, is  a  tint  of  red.  It  must  be  borne  in  mind,  how- 
ever, that  a  pure  color  is  not  necessarily  a  bright  color,  nor 
is  a  bright  color  necessarily  a  pure  color.  Some  parts  of 
the  spectrum  have  so  weak  a  tone  that  they  can  be  recog- 
nized only  with  difficulty,  yet  they  are  pure  colors. 

The  third  color-constant  is  the  intensity,  or  brightness, 
by  which  is  meant  the  amount  of  light  coming  from  a  unit 
area  of  the  colored  object.  In  this  connection  we  can  speak 
of  a  "dark"  red  and  a  "bright"  red ;  in  both  cases  the  satur- 
ation may  be  complete,  but  the  bright  red  sends  more  red 
light  into  our  eye  than  the  dark  red. 

Experiment  21.  On  the  color  top  combine  50  per  cent 
red  and  50  per  cent  black.  On  rotation,  a  very  dark  red  is 
seen;  this  is  called  a  shade  of  red.  Instead  of  having  the 
whole  amount  of  red  light  from  the  red  disk  thrown  on 
your  retina,  you  only  receive  one-half  of  this  amount  and, 
consequently,  the  intensity,  or  brightness,  is  less.  The  last 
two  experiments  may  be  repeated  with  blue ;  in  the  one  case 
a  tint,  in  the  other  a  shade  of  blue  is  obtained. 

Speaking  of  the  brightness  or  intensity  of  colors,  leads  us 


194  "T^^^    PHYSIOLOGY    OF    VISION. 

to  ask  what  is  the  brightest  part  of  the  spectrum.  Most 
people  agree  that,  with  ordinary  ilkimination,  the  yellow 
has  the  greatest  luminosity.  If,  however,  the  intensity  of  the 
light  is  decreased,  the  brightest  portion  of  the  spectrum 
shifts  over  to  the  right,  that  is,  into  the  green.  On  the 
other  hand,  if  the  intensity  of  the  decomposed  light  is  in- 
creased, the  brightest  part  is  found  in  the  orange.  This 
gives  rise  to  what  is  known  as  Purkinje's  phenomenon. 
While  looking  at  a  carpet  or  wall  paper,  the  pattern  of 
which  contains  red  and  blue  colors,  gradually  decrease  the 
light;  the  red  colors  gradually  become . darker  and  are  in- 
distinguishable from  black,  at  a  time  when  the  blue  can  be 
readily  discerned.  When  the  intensity  of  the  light  is  in- 
creased, the  colors  also  become  colorless,  but  the  order  in 
which  they  disappear  is  not  the  same  as  when  the  light  is 
decreased. 

We  have  seen  that  the  white  sunlight  can  be  decomposed 
into  its  component  colors,  red,  orange,  yellow,  etc.  If  we 
combine  these  colors  in  the  proper  proportion  we  again  have 
white  light.  This  can  be  done  on  the  color  top,  but  in  all 
these  demonstrations  with  the  color  top  you  must  bear 
in  mind  that  pure  white  is  never  obtained ;  instead  of  white, 
the  result  is  gray,  but  gray  is  a  white  of  less  intensity. 
Experiment  22.     On  the  color  top  mix  the  following : 

Red  12% 

Orange   11% 

Yellow    21% 

Green    24% 

Blue    18% 

Violet    14% 

A  dark  gray  is  obtained  in  Experiment  22,  indicating  that 
if  these  various  colors  are  combined  in  the  proper  propor- 
tions, the  effect  is  the  same  as  we  see  them  in  the  sunlight. 
However,  to  produce  white  light,  it  is  not  necessary  to  take 
all  these  colors.  White  light  can  be  produced  by  the  proper 
mixing  of  only  two  or  three  colors. 


THE    PHYSIOLOGY    OF    VISION.  I95 

Experiment  23.     On  the  color  top  mix: 

Green    31% 

Violet    34% 

Red   35% 

or 

Blue    25% 

Green    31% 

Red  44% 

This  experiment  indicates  that  the  sensation  of  white 
can  be  produced  by  the  admixture  of  ether  vibrations  of 
various  frequency.  By  mixing  50  per  cent  blue  and  50 
per  cent  yellow,  a  gray  is  also  produced.     Colors  which  on 


^''^^/''-'r:^ 


dU^f* 


Fig.  38.    Color  triangle. 

mixing  produce  white  light  are  called  complementary  colors, 
sometimes  also  called  contrast  colors.  The  colors  of  the 
spectrum  can  be  arranged  in  a  diagram,  as  illustrated  in 
Fig.  38.  This  is  a  triangle  with  rounded  corners,  at  which 
are  located  red,  green,  and  violet.  Between  them  are  the 
colors  in  the  order  in  which  they  are  found  in  the  spectrum. 
Near  the  center  of  the  triangle  we  have  white.  If  you  draw 
a  straight  line  through  the  white,  the  colors  that  are  located 
at  the  two  ends  of  this  line  are  complementary  colors.  For 
example,  take  the  line  which  connects  red  and  white ;  on 
prolonging  this  line,  it  cuts  the  triangle  between  green  and 
blue.  Here  is  located  the  bluish  green  of  the  spectrum,  and 
this  is  the  complementary  color  of  red. 

Experiment  24.     On  color  top  combine  red  44  per  cent, 
green  31  per  cent,  and  blue  25  per  cent;  the  result  is  gray 


196  THE    PHYSIOLOGY    OF    VISION. 

(white).  Now  mix  green  and  blue  in  the  same  proportion 
as  found  in  their  combination  with  red,  i.  e.,  mix  44  per  cent 
blue  and  56  per  cent  green.  This  gives  a  bluish  green  which 
is  complementary  color  of  red. 

In  a  similar  manner  it  can  be  seen  from  the  diagram  that 
yellow  and  indigo  are  complementary.  Again,  orange  and 
cyan  blue  (a  greenish  blue),  and  violet  and  greenish  yellow 
form  two  pairs  of  complementary  colors.  In  this  manner 
we  have  found  the  complementary  color  of  all  the  colors  of 
the  spectrum  except  green.  If  we  draw  a  line  from  green 
through  white,  it  cuts  the  broken  line  joining  the  red  and 
violet.  No  spectral  color  is  here  located,  but  on  this  line 
is  located  a  color  not  found  in  the  spectrum.  Can  we  deter- 
mine what  this  color  is  ?  Yes ;  in  the  following  way.  If 
two  colors,  separated  by  a  third  color,  are  mixed,  the  result 
is  the  third  color. 

Experiment  25.  With  the  large  disks  of  the  color  top 
mix  9  per  cent  yellow  and  91  per  cent  red,  and  with  the 
small  disks  mix  37  per  cent  orange  and  63  per  cent  black; 
rotate  these  simultaneously  on  the  top.  The  result  in  both 
disks  is  a  dark  shade  of  orange. 

Yellow  and  red  are  two  colors  separated  in  the  spectrum 
by  a  third  color,  orange,  and  the  mixing  of  these  two  colors 
produced  this  intermediate  color.  Now  red  and  violet  are 
separated  by  a  third  (unknown)  color,  to  determine  this 
color  we  need  only  mix  red  and  violet. 

Experiment  26.  Combnie  50  per  cent  red  and  50  per  cent 
violet.  A  beautiful  purple  is  the  result,  a  color  which  does 
not  exist  in  the  spectrum. 

Experiment  27.  As  no  purple  papers  are  found  in  the 
color  top  outfit,  we  must,  in  its  stead,  use  those  colors  that 
by  their  mixing  produce  purple,  viz.,  red  and  violet.  On 
wheel  mix  31  per  cent  green,  34  per  cent  violet,  and  35  per 
cent  red.  The  result  is  a  gray  (white),  proving  that  purple 
and  green  are  complementary  colors. 


LECTURE    VII. 

In  our  last  lecture  we  discussed  color  mixing  with  spe- 
cial reference  to  complementary  colors.  We  showed  that 
by  the  proper  mixing  of  two  or  three  colors,  white  light 
can  be  produced;  such  colors  are  called  complementary 
colors.  In  this  lecture  we  shall  continue  the  subject  of 
color  mixing  and  state  one  or  two  theories  which  seek  to 
explain  these  results. 

It  can  readily  be  demonstrated  that  it  is  possible  to  pro- 
duce white  light  and  all  the  colors  found  in  the  spectrum 
by  the  proper  mixture  of  only  three  colors.  These  three 
colors  are  called  the  primary,  or  fundamental,  colors.  The 
selection  of  these  three  colors  is  somewhat  arbitrary,  but 
generally  red,  green,  and  violet  are  taken.  By  the  proper 
mixing  of  these  primary  colors  any  color  and  white  can 
be  produced,  as  we  shall  prove  by  the  following  experi- 
ments. It  will  be  remembered  that  the  spectrum  is  divided, 
broadly  speaking,  in  the  seven  main  colors :  Red,  orange, 
yellow,  green,  blue,  indigo,  and  violet.  Of  these  we  are 
able  to  produce  orange,  yellow,  blue,  and  indigo  by  the 
proper  mixing  of  the  primary  colors. 

Experiment  28.  On  the  color-top  combine  28%  green 
and  ^2%  red.  A  shade  of  yellow  corresponding  quite  closely 
to  yellow  shade  No.  2*  is  produced.  Now  with  the  large 
disks  mix  16%  green  and  84%  red  and  with  the  small  disk 
25%  orange  and  75%  black;  rotate  these  simultaneously  on 
the  color-top;  a  dark  shade  of  orange  is  produced  by  both 
disks.  By  mixing  26%  green  and  74%  violet,  a  light  blue 
is  obtained. 


*  The  Milton  Bradley  Co.,  Springfield,  Mass.,  issue  a  small  book  of 
sample  colors  (paper).  The  colors,  shades,  and  tints  referred  to  in  this 
lecture  are  taken  from  this  booklet. 

197 


198 


THE    PHYSIOLOGY    OF    VISION. 


In  this  experiment  we  have  formed  yellow,  orange,  and 
blue  from  the  three  primary  colors.  In  the  same  way  we 
can  form  the  intermediate  colors.  We  may  say  in  general 
that  every  color  sensation  and  white  and  gray  can  be  pro- 
duced by  the  proper  mixing  of  other  color  sensations;  in 
fact,  certain  color  sensations,  e.  g.,  purple,  can  be  pro- 
duced only  by  the  mixing  of  other  color  sensations. 

Many  theories  have  been  advanced  to  explain  color 
vision.     The  Helmholtz  theory  states  there  are  three  fibres 


Fig.  39. 


0       Y         C  3  V 

Diagram  of  tlie  three  color  sensations.— Helmholtz. 


(or  substances)  in  the  eye;  a  red  fibre,  a  green  fibre,  and  a 
violet  fibre  (Fig.  39).  The  red  fibre  (upper  curve  of  Fig. 
39)  is  stimulated  very  strongly  by  the  red  light,  to  a  slight 
extent  by  the  green,  and  very  slightly  by  the  violet.  The 
green  fibre  is  slightly  stimulated  by  both  red  and  violet, 
but  to  a  much  greater  extent  by  green  light.  The  violet 
fibre  is  stimulated  most  strongly  by  the  violet  light,  less 
by  the  green  light,  and  least  of  all  by  the  red  light. 

It  will  be  noticed  that  all  three  fibres  are  acted  upon  by 
all  three  of  the  primary  colors.  How  are  the  various  color 
sensations  produced?  The  sensation  of  red  is  produced 
when  the  red,  the  green,  and  the  violet  fibres  are  stimulated 
to  the  relative  extent  of  ab,  cd,  and  ef  respectively   (Fig. 


THE    PHYSIOLOGY    OF    VISION.  1 99 

39).  In  a  similar  manner  it  is  evident  from  Fig.  39  that 
a  sensation  of  green  originates  by  the  simultaneous  stim- 
ulation of  the  red,  green,  and  violet  fibres  to  the  extent  of 
gh,  ij,  and  kl  respectively.  How  does  this  theory  explain 
the  perception  of  colors  other  than  the  primary  colors? 
How,  for  example,  is  the  sensation  of  orange  produced? 
Orange  is  produced  by  the  stimulation  of  the  red  fibre  to 
the  extent  of  mn,  by  the  stimulation  of  the  green  fibre  to 
the  extent  of  op,  and  by  the  stimulation  of  the  violet  to  the 
extent  of  qr ;  the  last  quantity  is  so  small  that  for  all  prac- 
tical purposes  we  may  neglect  it.  Now  experiment  28  has 
shown  us  that  by  mixing  16%  green  plus  84%  red,  a  dark 
orange  is  obtained.  This  is,  therefore,  in  harmony  with 
the  Helmholtz  theory.  Again,  from  Fig,  39,  it  can  be  seen 
that  to  produce  the  sensation  of  yellow,  red  and  green  must 
again  be  mixed  (neglecting  the  small  amount  of  violet),  but 
that  the  amount  of  green  must  be  greater  while  the  amount 
of  red  must  be  less  than  that  needed  for  the  production  of 
orange.  Experiment  28  demonstrated  that  this  is  correct, 
for  to  produce  yellow  we  used  28%  green  plus  72%  red. 
Again,  Fig.  39  indicates  that  blue  ought  to  be  produced  by 
the  stimulation  of  the  green  and  violet  fibres  (neglecting 
the  small  amount  of  red).  Here  again  experiment  28 
agrees  with  the  Helmholtz  theory.  According  to  this 
theory,  if  all  three  fibres  are  stimulated  to  the  same  ex- 
tent, the  sensation  is  white;  experiment  23  has  shown  us 
that  by  mixing  violet,  red,  and  green,  a  gray  is  obtained. 
Hence  the  Helmholtz  theory  is  true  here  also. 

Another  theory  of  color  sensation  is  known  as  Hering's 
theory.  The  Hering  theory  of  color  sensation  postulates 
the  existence  of  three  substances  in  the  eye,  a  red-green,  a 
yellow-blue,  and  a  white-black  substance.  When  light  falls 
upon  the  retina,  these  substances  are  broken  down  or  built 
up  in  the  following  manner.  Red  light  decomposes  the  red- 
green  substance,  while  green  light  builds  it  up ;  yellow  light 
causes  the  breaking  down  of  the  yellow-blue  substance,  and 


200  THE    PHYSIOLOGY    OF    VISION. 

blue  light  causes  it  to  be  built  up.  All  light,  no  matter  of 
what  color,  causes  decomposition  of  the  white-black  sub- 
stance, this  substance  being  regenerated  in  the  absence  of 
light.  Complementary  colors  are  explained  by  this  theory 
in  the  following  manner:  Yellow  and  blue,  as  we  have 
seen,  are  complementary;  if  mixed,  white  is  produced. 
The  yellow  light  causes  decomposition,  the  blue  light  causes 
building  up  of  the  yellow-blue  substance.  If  the  destruc- 
tive and  constructive  processes  are  equal,  the  yellow-blue 
substance  undergoes  no  change,  and  consequently  no  color 
is  perceived.  But  both  yellow  light  and  blue  light  cause 
decomposition  of  the  white-black  substance,  and  this  pro- 
duces the  sensation  of  white.  Hence  the  proper  mixture  of 
yellow  and  blue  causes  the  sensation  of  white. 

Thus  far  we  have  discussed  the  combination  of  comple- 
mentary colors;  what  happens  if  colors  are  mixed  that  are 
not  complementary?  This  may  be  answered  by  the  fol- 
lowing experiments. 

Experiment  29.  On  the  color-top  mix  green  and  blue  in 
various  proportions.  It  will  be  noticed  that  the  resulting 
sensation  is  either  a  greenish  blue  or  a  bluish  green;  no 
other  color  sensation  can  be  produced.  Blue  and  green 
are  neighboring  colors  in  the  spectrum  (Fig.  38),  and  the 
colors  originated  by  their  mixing  always  lie  between  these 
two  colors,  and  are  found  as  such  in  the  spectrum.  The 
same  can  be  demonstrated  by  combining  yellow  and  green 
in  various  proportions. 

Suppose  the  colors  lie  further  apart,  what  is  the  result? 
Experiment  25  has  taught  us  that  if  two  colors,  separated 
by  a  third  color,  are  combined  in  the  correct  proportion, 
the  intermediate  color  is  produced.  For  example,  red  and 
yellow  (separated  by  orange)  on  mixing  produce  orange. 
Again,  suppose  the  colors  are  still  further  apart.  In  gen- 
eral we  may  state  that  if  the  two  combining  colors  are  sit- 
uated closer  together  than  their  complementary  colors,  the 
resulting  color  lies  between  the  two  colors  that  are  com- 


THE    PHYSIOLOGY    OF     VISION.  20I 

bined,  but  if  they  are  situated  further  than  the  comple- 
mentary colors,  the  resulting  color  lies  outside  of  the  com- 
plementary colors  and  resembles  more  or  less  a  purple. 

Experiment  30.  Combine  green  and  red  in  various  pro- 
portions; the  result  is  a  yellow  or  orange.  The  comple- 
mentary color  of  red  is  bluish-green;  hence  the  two  colors 
mixed  in  this  experiment  lie  closer  together  than  the  com- 
plementary colors  (see  Fig.  38),  and  the  result  is  an  in- 
termediate color,  either  yellow  or  orange.  The  same  can 
be  proved  by  combining  violet  and  green,  which  are  closer 
together  than  the  complementary  colors  (violet  and  green- 
ish yellow).  The  result  is  a  greenish  blue,  a  bluish  green, 
a  blue,  or  a  bluish  violet,  depending  on  the  proportion  of 
green  and  violet  combined. 

Experiment  31.  Combine  red  and  blue  in  the  following 
proportion : 

a.  50%  blue  plus  50%  red  equals  violet. 

b.  25%  blue  plus  75%  red  equals  purple. 

The  two  colors  here  combined  are  further  apart  than  the 
complementary  colors  (red  and  bluish  green)  and  the  re- 
sulting colors,  violet  or  purple,  lie  outside  of  the  comple- 
mentary colors  (compare  Fig.  38).  The  same  can  be 
demonstrated  by  combining  43%  violet  plus  57%  orange, 
which  gives  a  very  beautiful  red  (tint  No.  2)  ;  this  lies  out- 
side of  the  complementary  colors  (violet  and  greenish  yel- 
low). 

If  two  color  sensations  are  identical  and  both  of  them 
are  altered  to  the  same  extent,  the  resulting  sensations  are 
again  identical.  This  is  strikingly  brought  out  in  the  fol- 
lowing experiment: 

Experiment  32.  With  large  disks  mix  16%  green  and 
84%  red,  and  with  smaller  disks,  25%  orange  plus 
75%  black.  In  both  disks  a  dark  orange  is  seen.  Now  in- 
troduce 20%  violet  in  both  the  larger  and  smaller  disks. 
In  order  to  keep  the  proportions  true,  it  is  evident  that  the 
amounts  of  the   red   and  green   in  the  larger  and  of  the 


202  THE    PHYSIOLOGY    OF    VISION. 

orange  and  black  in  the  smaller  disk  must  be  decreased. 
The  following  gives  the  correct  proportions : 

Larger  disk: — 20%  violet  plus  I2i^%  green  plus  djYzP/o 
red. 

Smaller  disk: — 20%  violet  plus  20%  orange  plus  60% 
black. 

Place  these  two  disks  on  the  color  top  and  the  sensa- 
tions produced  by  them  are  identical — a  dark  red,  nearly 
identical  with  the  "A-Red  dark"  of  the  broken  spectrum 
scale. 

The  foregoing  experiment  demonstrates  clearly  that  if 
two  equal  sensations  are  produced  by  two  different  phys- 
ical stimuli,  altering  both  stimuli  equally,  also  alters  the 
sensations  equally,  and  hence  the  sensations  remain  equal. 
Before  we  dismiss  the  subject  of  color  mixing,  I  may  be 
permitted  to  state  one  more  interesting,  although  somewhat 
complicated,  experiment. 

Experiment  33.     Combine — 

A.  20%  orange  plus  80%  blue  equals  violet. 

B.  50%  red  plus  50%  violet  equals  purple. 

In  equation  B,  instead  of  using  50%  violet  we  can  use 
the  value  of  violet  found  in  equation  A,  that  is,  20%  orange 
plus  80%  blue.    The  equation  then  becomes : — 

C.  50%  red  plus  10%  orange  plus  40%  blue  equals  purple. 
Place  this  on  the  small  disk,  while  on  the  large  disk  you 

have  B.  Both  C  and  B  produce  the  same  purple.  Again, 
red  can  be  produced  by  mixing : — 

D.  36%  blue  plus  64%  orange  equals  red  (tint  No.  2). 

We  can  substitute  this  value  of  red  in  equation  C  and  ob- 
tain (by  taking  one-half  of  the  blue  and  of  the  orange 
of  D)  :— 

E.  18%  blue  plus  32%  orange  plus  10%  orange  plus  40% 
blue  equals  purple. 

Simplifying  this  by  combining  like  factors  we  have : — 

F.  58%  blue  plus  42%  orange  equals  purple  (called  violet 
red,  tint  No.  2,  in  Bradley's  Color  book.) 


THE    PHYSIOLOGY    OF    VISION.  203 

If  now  B  and  F  are  rotated  simultaneously  on  the  color 
top  the  two  resulting  colors  are  nearly  identical.* 

There  is  still  one  point  to  which  I  must  call  attention,  be- 
cause it  will  remove  certain  objections  that  may  be  raised  in 
some  minds  against  the  theory  of  color  mixing  as  here  out- 
lined. I  refer  to  the  mixing  of  pigments.  It  is  well  known 
to  artists  that  the  mixing  of  blue  and  yellow  paint  produces 
green.  Yet  we  have  called  these  colors  complementary  col- 
ors, that  is,  by  their  mixture  they  produce  white.  This  dif- 
ference can  readily  be  explained ;  but  before  proceeding  to 
this,  let  me  make  the  following  observation.  When  we  com- 
bine colors  on  the  color  top  we  do  not  combine  colors,  but 
color  sensations.  In  fact,  colors  have  no  objective  exist- 
ence; colors  are  psychological  phenomena.  What  corre- 
sponds to  colors  are  ether  vibrations  of  different  lengths. 
For  example,  when  we  view  the  yellow  in  the  spectrum,  the 
eye  is-  stimulated  by  ether  waves  having  a  length  of  0.00058 
mm. ;  when  we  view  the  blue,  the  eye  receives  ether  waves 
having  0.00047  mm.  length.  We  can  let  these  two  waves 
fall  into  the  eye  simultaneously  and  the  result  is  a  sensation 
of  white.  It  must  not  be  imagined  that  the  waves  of  0.00047 
mm.  and  0.00050  mm.  length  have  combined  or  fused; 
what  has  been  combined  is  the  sensation  of  yellow  and  the 
sensation  of  blue,  and  this  results  in  an  entirely  new  sensa- 
tion, viz.  white.  To  explain  this  combining  of  sensations, 
theories  of  color  vision,  such  as  the  Helmholtz  or  the  Hering 
theory,  are  put  forward. 

This  happens  when  two  colors  are  simultaneously  thrown 
into  the  eye,  as  is  done  by  means  of  the  color  wheel.  What 
happens  when  yellow  and  blue  pigments^are  mixed  ?  Yellow 
pigment  is  yellow  because  it  absorbs  all  the  light  rays  ex- 

*B  and  F  are  not  absolutely  identical.  The  sensation  of  B  is 
equivalent  to  that  called  violet  red  in  the  Bradley's  color  booklet; 
while  F.  as  above  stated  is  equivalent  to  violet  red  tint  No.  2.  This  is 
due  to  the  fact  that  in  D  the  blue  and  orange  did  not  produce  saturated 
red,  but  red  tint  No.  2.  If  it  were  possible  by  these  disks  to  produce  a 
saturated  red,  the  result  in  B  and  F  would  be  identical.  I  would  sug- 
gest that  if  a  color  wheel  with  disks  in  three  sizes  is  accessible  the  ex- 
perimenter compare  B,  C  and  F,  by  rotating  them  sinmltaneously. 
The  result  is  very  striking. 


204  THE    PHYSIOLOGY    OF    VISION. 

cept  the  yellow  and  some  of  the  green,  it  reflects  these  into 
the  eye.  Blue  pigment,  on  the  other  hand,  absorbs  all  ex- 
cept the  blue  and  some  of  the  green;  hence  when  the  two 
are  combined,  the  yellow  pigment  absorbs  the  blue  reflect- 
ed by  the  blue  pigment,  the  blue  pigment  absorbs  the  yellow 
reflected  by  the  yellow  pigment,  but  both  reflect  the  green. 
Consequently  by  mixing  them  there  is  no  combining  of 
color  sensations  in  the  eye  or  brain,  but  there  is  an  elimina- 
tion of  ether  waves. 

After  this  lengthy  discussion  of  color  mixing  we  shall 
proceed  with  the  subject  of  color-blindness.  Color-blindness 
is  the  inability  to  discriminate  between  certain  colors  which 
the  normal  eye  finds  no  difficulty  in  distinguishing.  The 
number  of  color-blind  people  is  stated  at  about  three  or  four 
per  cent  in  the  male  and  one-fourth  of  one  per  cent  in  the 
female.  Color-blindness  may  be  inherited;  the  daughter  of 
a  color-blind  person  may  have  normal  color  vision-,  but 
have  sons  who  inherit  the  grandfather's  defect.  Strange  as 
it  may  appear,  color-blindness  may  be  monocular,  i.  e.,  may 
exist  in  one  eye  only,  the  other  eye  having  normal  color 
vision.  As  in  one  form  of  color-blindness  green  and  red 
are  confused,  it  is  of  the  greatest  importance  that  employes 
of  railways  and  steamboats  be  carefully  examined  as  to  their 
color  vision,  seeing  that  red  and  green  signals  are  used  to 
indicate  safety  or  danger,  etc. 

One  formi  of  color-blindness,  which  is  very  rare,  is  achro- 
motopsia,  in  which  no  colors  whatever  are  perceived ;  only 
white,  black,  and  the  various  shades  of  gray  are  seen.  To 
such  people  a  painting  appears  as  an  engraving.  The 
most  common  form  of  color-blindness  is  red-green  blindness 
in  which,  as  already  stated,  red  and  green  are  confused. 
Some  hold  there  are  two  classes  of  red-green  blindness,  the 
red-blind  and  the  green-blind;  others  say  there  is  but  one 
class,  but  that  the  color  vision  varies  somewhat  in  diiferent 
color-blind  individuals.  We  have  not  the  time  to  enter  into 
the  details  of  color-blindness  and   shall   very  briefly  state 


THE    PHYSIOLOGY    OP^    VISION.  205 

the  most  important  points.  The  red-blind  mix  up  Hght 
red  with  dark  green;  to  the  green-blind  light  green  and 
dark  red  appear  identical.  The  red  part  of  the  spectrum 
is  invisible  to  the  red-blind,  while  the  green-blind  is  said 
to  see  the  spectrum  in  its  normal  length.  To  both  the  red- 
blind  and  green-blind  the  yellow  and  the  blue  are  said  to 
be  normal.  In  fact,  all  the  colors  which  they  are  able  to 
perceive  can  be  produced  by  the  proper  mixing  of  two 
colors,  yellow  and  blue.  It  will  be  remembered  that  for 
the  normal  individual  all  colors  can  be  produced  by  mixing 
three  colors,  viz.,  red,  green,  and  blue  (or  violet).  Hence, 
the  normal  color  vision  is  said  to  be  trichromatic,  while 
the  vision  of  the  red-green  blind  is  dichromatic. 

The  method  which  is  now  generally  employed  to  detect 
red-green  "blindness  is  the  "Holmgren  method."  In  the 
Holmgren  method  a  pile  of  worsteds  of  various  colors, 
shades  and  tints  is  used;  the  examinee  is  given  a  "test- 
skein"  which  must  be  matched  by  tints  and  shades  of  the 
same  color  selected  from  the  pile  of  worsteds.  The  first 
test-skein  given  to  him  is  a  light  green.  If  his  color  vision 
is  normal  he  matches  this  only  by  green  of  various  tints 
and  shades.  If  he  is  red-green  blind,  he  selects  besides 
the  greens,  pink,  yellow,  orange,  red-grays,  and  pure  grays. 
He  is  then  handed  a  pale  rose  test-skein.  The  red-blind 
chooses  blue  and  violet,  while  the  green-blind  selects  gray 
and  green.  ,  Finally  a  bright  red  test-skein  is  given ;  the 
red-blind  selects,  in  addition  to  the  red,  green,  and  browns 
of  a  darker  shade  than  the  test-skein,  while  the  green-blind 
selects  light  greens  and  light  browns. 

As  there  are  various  degrees  of  color  blindness,  a  person 
may  fail  on  the  first  test  and  pass  the  second  and  third  tests 
successfully ;  he  is  then  said  to  be  incompletely  color-blind. 
Some  people  have  no  difficulty  in  distinguishing  between 
red  and  green  if  the  colors  are  viewed  in  full  light  or  at 
close  range,  while  they   fail  to   recognize  them  when  the 


206  THE    PHYSIOLOGY    OF    VISION. 

colors  are  placed  in  unfavorable  condition,  such  as  great 
distances,  dim  light,  etc. 

Dalton,  the  great  chemist,  was  red-green  blind ;  he  was  un- 
able to  find  his  scarlet  coat  on  the  green  grass.  As  he  was 
one  of  the  first  to  describe  this  defect,  red-green  blindness 
is  sometimes  called  Daltonism. 

Another  form  of  dichromatism  is  the  yellow-blue  blind- 
ness, in  which  all  the  colors  can  be  obtained  by  mixing  the 
two  colors  red  and  green.  Yellow  and  blue  are  confused 
with  green  or  red.  This  form  is  much  rarer  than  the  red- 
green  blindness. 

Various  explanations  have  been  given  for  color-blind- 
ness, depending  on  the  various  theories  of  color  vision.  In 
general  we  may  say  that  there  is  a  lack  of  the  color  per- 
ceiving element  in  the  eye  or  brain.  That  the  defect  is  per- 
haps located  in  the  eye  is  proved  by  the  fact  that  one  eye 
may  be  color-blind  while  the  other  eye  is  normal,  and  that 
by  heating  the  eyeball  of  a  color-blind  person  the  defect 
disappears  temporarily. 

Color-blindness  may  be  brought  about  by  disease,  such 
as  disease  of  the  optic  nerve,  and  by  the  abuse  of  tobacco 
and  certain  other  substances.  In  epilepsy  there  may  be 
intermittent  color-blindness.  As  I  said  a  little  while  ago, 
the  percentage  of  color-blindness  is  much  higher  in  men 
than  in  women.  Perhaps  this  is  due  to  the  better  training 
in  colors  that  girls  receive ;  if  this  is  true,  it  may  be  possible 
to  reduce  the  per  cent  of  the  color-blind  by  properly  edu- 
cating our  children.  It  is  claimed  that  color-blindness  is 
more  prevalent  among  savages  than  civilized  people. 
Recent  iuA'^estigations  in  Patagonia  have  proved  that  very 
many  of  the  savages  have  no  color  sensations  of  blue ;  blue 
and  black  appear  identical.  From  the  study  of  the  color 
terminology  of  Homer,  Gladstone  concluded  that  the  old 
Greeks  were  color-blind.  It  would  seem,  therefore,  that  the 
color  sense  is  a  recent  acquirement  of  the  human  race,  and 
this  is  borne  out  by  the  development  of  the  human  being,  a 


THE    PHYSIOLOGY    OF    VISION. 


207 


child  is  unable  to  distinguish  colors  till  toward  the  end  of 
the  second  year.  However,  as  many  lower  animals  give  in- 
disputable evidence  of  color  sense,  it  does  not  seem  likely 
that  this  faculty  should  be  lacking  in  primitive  man.* 

Certain   portions   of   our   retina   are   always  color-blind. 
The  fovea  centralis  may  have  normal  color  vision,  but  the 


Fig.  40.  Perimetric  chart  of  the  left  retina,  showing  the  extent  of  the 
retina  on  which  the  various  colors  can  be  perceived.  The  shaded  circle 
a  little  to  the  left  of  the  center  is  this  blind  spot, 


farther  we  proceed  to  the  periphery  of  the  retina,  the  less 
the  color  sense,  and  at  the  extreme  border  of  the  retina  we 
have  no  color  sense  whatever.  Fig.  30  is  a  map  of  the  left 
retina,  indicating  the  extent  of  the  retina  by  which  the  var- 
ious colors  can  be  perceived.  It  will  be  noticed  that  the 
field  for  green  is  extremely  limited,  while  that  for  blue  is 
the  greatest,  and  that  the  outer  portions  of  the  retina  are 
absolutely  color-blind. 


*  See  the  interesting  article  on  Primitive  Color  vision,  by  Dr 
W.  H.  R.  Rivers,  in  the  Popular  Science  Monthly,  May,  1901.  Vol.  LIX 
page  44. 


2o8  THE    PHYSIOLOGY    OF    VISION. 

Having  discussed  color  sensation  and  the  theories  ot 
color  vision,  we  are  now  ready  to  proceed  to  the  subject  of 
negative  after-images,  or  successive  contrast. 

Experiment  34.  Look  intently  for  one  or  two  minutes  at 
the  cross  in  Fig.  41.  Next  fix  your  gaze  on  a  small  mark 
on  a  white  sheet  of  paper.  A  negative  image  of  Fig.  41  is 
seen  in  which  the  upper  left  and  lower  right  hand  quarters 
are  black,  while  the  other  two  quarters  are  white,  hence 
the  reverse  of  the  original  figure. 


Fig.  41. 

This  after-image  is  called  the  negative  after-image  be- 
cause in  it  the  light  and  dark  and  also  the  colors  are  the 
reverse  (negative)  of  the  original  (positive)  ;  it  is  some- 
times called  successive  contrast,  because  it  is  a  contrast 
which  succeeds,  or  follows,  the  original  image. 

Experiment  35.  Upon  a  white  sheet  of  paper  place  a 
red  piece  of  paper  or  cloth  and  fix  your  eye  upon  a  certain 
spot  of  the  colored  object.  It  is  very  necessary  not  to  shift 
the  line  of  fixation.  After  one  or  two  minutes  look  at  a 
uniform  white  surface  and  a  bluish  green  negative  after- 
image is  seen.  Try  the  same  with  green,  yellow,  and  blue 
objects. 

From  this  experiment  it  is  evident  that  the  image  of  a 
colored  object  is  followed  by  a  negative  after-image  in 
which  the  original  colors  are  seen  in  the  complementary 
colors. 

The  explanation  of  the  formation  of  the  negative  after- 
image that  we  shall  give  is  based  on  the  Hering  theory  of 


THE    PHYSIOLOGY    OF    VISION. 


209 


color  vision.  At  the  beginning  of  this  lecture  we  stated 
that  Hering  postulated  the  existence  of  three  visual  sub- 
stances in  the  retino-cerebral  mechanism,  the  red-green, 
the  yellow-blue,  and  the  white-black  substances.  Suppose 
the  eye  is  exposed  to  yellow  light;  great  destruction  of  the 
yellow-blue  substance  takes  place.  The  red-green  sub- 
stance is  not  affected  and  the  white-black  is  broken  down 
to  a  small  extent.  This  last  effect  we  need  not  consider  at 
present.     After  the  yellow-blue  substance  has  been  broken 


Fig.  42. 


down  to  a  considerable  extent,  white  light  is  thrown  into 
the  eye.  The  red  and  green  of  the  white  light  cause 
simultaneous  and  equal  construction  and  destruction  of 
the  red-green  substance,  and  therefore  produce  no  color 
sensation.  The  yellow  light  in  the  white  light  normally 
causes  destruction  of  the  yellow-blue  substance,  but  as  this 
substance  has  previously  undergone  great  destruction,  the 
breaking  down,  when  white  light  is  looked  at,  is  extremely 
limited.  On  the  other  hand,  the  blue  light,  which  always 
causes  the  building  up  of  the  yellow-blue  substance,  now 
finds  plenty  of  material  to  build  up ;  hence  the  white  light 
does  not  appear  colorless  but,  owing  to  the  great  re- 
generation   of    the    yellow-blue    substance,   appears    blue. 


2IO  THE    PHYSIOLOGY    OF    VISION. 

This  is  the  negative  after-image  of  yellow.  In  a  similar 
manner  all  the  negative  after-images  obtained  in  experi- 
ment 35  can  be  explained. 

In  passing  we  might  mention  that  Helmholtz  explained 
the  origin  of  negative  after-images  as  due  to  fatigue.  This 
is  not  correct,  for  these  images  are  best  seen  in  the  morn- 
ing when  the  eye  is  least  susceptible  to  fatigue.  Young 
and  vigorous  persons  see  these  negative  after-images  better 
than  old  and  feeble  people. 

Negative  after-images  can  also  be  obtained  by  exposing 
the  eye  to  an  object  (like  Fig.  41)  and  then  closing  the 
eye.     Perhaps  this  is  due  to  the  intrinsic  light  of  the  eye 


Fig.  43 

which  we  have  mentioned  before.  We  might  ask  the 
question,  are  the  negative  after-images  due  to  processes 
taking  place  in  the  brain  or  in  the  eye?  This  cannot  be 
answered  satisfactorily.  It  is  true,  as  one  can  prove  by 
experiment,  that  a  sudden  change  in  the  accommodation  of 
the  eye,  or  a  sudden  movement  of  the  eyeball,  causes  the 
image  to  disappear.  If  the  eyeball  is  mechanically  dis- 
placed, as  by  pushing  it  with  the  finger,  the  negative  after- 
image also  moves.  These  facts  indicate  that  the  develop- 
ment of  the  negative  after-image  depends  on  changes  oc- 
curing  in  the  eye  itself;  however,  the  question  is  not  set- 
tled. The  following  experiment  is  also  due  to  negative 
after-images. 

Experiment  36.  With  the  right  eye  look  at  a  red  object 
(paper,  cloth)  for  one  or  two  minutes;  on  now  looking  at 
a  violet  color,  this  appears  blue,  as  can  be  seen  by  quickly 


THE    PHYSIOLOGY    OF    VISION. 


21  I 


closing  the  right  eye  and  opening  the  left  eye.  If  the  eye 
is  first  exposed  to  yellow  light,  orange  appears  reddish- 
orange.  Yellow  has  a  greenish  yellow  appearance  if  the 
eye  is  previously  stimulated  by  orange  light. 

There  is  another  contrast  which  is  called  simultaneous 
contrast  because  it  occurs  simultaneously  with  the  viewing 
of  the  complementary  color. 


^^*°. 


Fig.  44 


Experiment  ^^j.  On  viewing  the  gray  V-shaped  figures 
in  Fig.  42,  the  V  on  the  black  field  appears  lighter  than 
the  gray  on  the  white  field,  although  both  have  the  same 
intensity.    This  is  due  to  contrast. 

Experiment  38.  On  a  white  and  on  a  black  field  as 
shown  in  Fig.  42  place  small  squares  of  red,  yellow,  green, 
and  blue  paper.  The  colors  on  the  black  field  are  con- 
siderably brighter  than  those  on  the  white  field.  A  color 
in  a  dark  setting  appears  brighter  and  livelier  than  in  a 
bright  setting. 


212  THE    PHYSIOLOGY    OF    VISION. 

Experiment  39,  On  a  piece  of  yellow  paper  paste  nar- 
row strips  of  gray  paper  (about  1-12  or  1-8  inch  wide). 
On  covering  the  whole  by  a  piece  of  white  tissue  paper,  the 
gray  strips  appear  bluish.  If  gray  strips  are  placed  on 
green  paper,  the  strips  look  reddish.  The  color  of  the 
background  induces  the  complementary  color  in  the  gray. 
This  is  also  shown  in 

Experiment  40.  On  a  color  wheel  rotate  a  disk  such  as 
represented  in  Fig.  43.  The  disk  is  made  of  white  paper, 
the  shaded  sectors  are  made  of  colored  paper  and  the 
black  squares  in  the  colored  sectors  are  pieces  of  black 
paper.  Suppose  that  the  colored  paper  is  red ;  on  rotating, 
the  ring  occupied  by  the  black  squares  is  not  gray  as  one 
would  expect,  but  it  appears  in  the  complementary  color, 
greenish  blue.  Whatever  color  may  be  used,  the  ring  is 
always  seen  in  the  complementary  color.  We  may  state 
that  this  experiment  does  not  succeed  very  well  in  artificial 
light. 

One  more  experiment  before  I  close  this  lecture. 

Experiment  41.  On  the  table  place  a  sheet  of  white  pa- 
per (F  in  Fig.  44)  illuminated  by  feeble  daylight  (from 
the  window  A,  Fig.  44).  Have  a  lighted  candle  or  a  lamp, 
C,  in  such  a  position  that  the  shadow,  P,  of  an  object,  O 
(a  lead  pencil,  for  example),  is  cast  on  the  paper.  Reg- 
ulate the  amount  of  daylight  falling  on  the  paper  so  that 
the  shadow  P  is  dim;  it  appears  bluish.  This  is  because 
the  lamplight  is  not  white  but  yellow.  The  light  from  the 
paper  illuminated  by  the  candle  is  yellow,  that  is,  Q  and  Q 
in  Fig.  44  are  yellow.  The  light  received  from  the  shadow 
is  white  but  the  neighboring  yellow  induces  a  blue  color. 
This  experiment  can  be  varied  by  using  colored  glass, 
red  or  green,  so  that  the  field  is  illuminated  not  only  by 
the  white  daylight  but  also  by  the  colored  light.  The  shad- 
ows will  be  seen  in  the  complementary  colors. 

These  experiments  on  simultaneous  contrast  indicate 
that  the  sensation  resulting  from  the  stimulation  of  a  cer- 


THE    PHYSIOLOGY    OF    VISION.  213 

tain  portion  of  the  retina  depends  not  only  upon  the  nature 
of  the  stimulation  but  also  upon  the  condition  of  neighbor- 
ing parts  of  the  retina.  To  use  the  phraseology  of  Her- 
ing's  theory,  if  destruction  of  the  yellow-blue  substance 
takes  place  in  a  certain  portion  of  the  retina  (whereby  the 
sensation  of  yellow  is  obtained),  in  the  adjoining  part  of 
the  retina  the  opposite  process,  that  is,  the  building  up  of 
the  ycllow-blue  substance,  may  occur  and  the  resulting 
sensation  is  blue. 


LECTURE  VIII. 

At  the  beginning  of  the  second  lecture  we  stated  that  the 
fcnrth  requisite  for  vision  is  the  projection  of  the  sensation 
into  space.     This  subject  will  occupy  us  in  our  last  lecture. 

As  we  stated  in  the  first  lecture,  the  images  on  the  retina 
are  inverted.  Yet  we  interpret  these  images  correctly,  that 
is,  we  "re-invert"  them  so  that  we  see  the  object  in  its  cor- 
rect position.  How  this  is  accomplished  is  difficult  to  say. 
We  are  ordinarily  not  conscious  of  the  so-called  special 
sense  organs,  like  the  ear  and  eye,  when  they  are  stimulated, 
but  always  refer  or  project  the  sensation  produced  by  their 
stimulation  into  the  outer  world.  This  is  not  true  for  all 
our  sensations.  When,  for  example,  a  knife  cuts  through 
the  skin,  we  do  not  think  of  the  knife  but  of  the  seat  of  the 
pain ;  in  other  words,  we  project  the  sensation  of  pain  to  a 
certain  part  of  our  own  body  and  not  to  the  outer  world. 
To  a  certain  extent  this  is  also  true  for  the  sensations  of 
heat  and  cold.  If  our  feet  rest  upon  a  cold  piece  of  iron, 
we  generally  project  the  sensation  into  the  outer  world  and 
are  conscious  of  a  cold  object;  we  think  of  the  cold  as  re- 
siding in  the  foreign  object,  if  I  may  use  this  expression. 
But  when  a  gentle  stream  of  cold  air  comes  in  contact  with 
our  feet,  we  seldom  think  of  the  cold  air  but  we  say  our 
feet  are  cold. 

Our  visual  sensations  are  always  projected  into  space, 
even  when  the  cause  of  the  stimulation  resides  in  the  eye 
itself,  as  is  seen  in  the  intrinsic  light  of  the  retina. 
This  projection  is  not  at  hap-hazard,  but  follows  a 
definite  law  so  that  our  hand,  guided  by  our  visual  sensation, 
can  be  laid  upon  the  object  seen.  The  law  of  this  projection 
is  that  we  project  the  sensation  into  the  outer  world  along 

214 


THE    PHYSIOLOGY    OF    VISION.  215 

the  line  which  joins  the  image  formed  on  the  retina  with 
the  nodal  point  of  the  eye.  Of  the  many  examples 
proving  this,  we  have  time  to  call  attention  to  only  one 
or  two. 

Experiment  42.  Press  the  outer  corner  of  the  right  eye 
with  the  tip  of  the  fmger.  A  phosphene  is  seen  (see  ex- 
periment 12)  and  it  will  be  noticed  that  this  phosphene  is 
situated  to  the  left.  Suppose,  in  Fig.  45,  the  finger  is  ap- 
plied to  the  eye  at  a.  This  portion  of  the  retina  is  mechanic- 
ally stimulated  and  a  sensation  is  produced.  The  nodal 
point  of  the  eye  is  at  n  ;  the  sensation  produced  by  the  stimu- 
lation at  a  is  projected  in  the  direction  of  the  line  anx  and 
hence  the  phosphene  appears  on  the  side  opposite  to  the 
point  of  stimulation. 
X 


'ig.  45 


Another  proof  of  this  law  is  seen  in — 
Experiment  43.  '  Close  the  left  eye  and  with  the  other 
eye  look  at  an  object.  Suddenly  place  a  prism  before  the 
eye  in  such  a  manner  that  the  base  (thickest  part  of  the 
prism)  is  toward  the  right.  The  object  now  appears  dis- 
placed, as  can  readily  be  seen  by  turning  the  prism  slowly 
around  a  horizontal  axis.  When  the  base  of  the  prism  is 
to  the  right  (temporal  side),  the  object  seems  to  be  located 
more  tolJhe  left  of  the  experimenter;  when  the  base  of  the 
I^rism  is  held  toward  the  nose,  the  object  appears  to  be 
situated  on  the  temporal  side. 

'  The  reason  for  this  is  as  follows :  In  Fig.  46  let  a  be  the 
object  looked  at  with  the  right  eye.  The  object  sends  one 
ray  of  light  throtigh  the  nodal  point  (n),  this  ray  is  not 
refracted   (broken  line  in  Fig.  46)   and  the  image  of  the 


2l6 


THE    PHYSIOLOGY    OF    VISION. 


object  lies  where  this  h'ne  meets  the  retina  (at  b).  The 
sensation  produced  is  referred  from  b  through  n  to  the 
outer  world.  Now  place  the  prism  in  front  of  the  eye  so 
that  its  base  is  toward  the  right  (temporal  side)  ;  the  ray  of 
light  in  passing  through  the  prism  is  refracted  toward  the 
base  of  the  prism  so  that  it  now  strikes  the  cornea  at  c,  and 
after  refraction  in  the  eye  stimulates  the  retina  at  d.  The 
sensation  produced  at  d  is  projected  through  the  nodal  point 
(n),  hence  the  object  appears  to  be  situated  at  a\     If  the 


Fig.  47 


base  of  the  prism  is  placed  to  the  left,  the  object  appears  to 
be  displaced  to  the  right. 

It  is  for  this  reason  that  the  pin  in  experiment  6  appears 
to  move  in  a  direction  contrary  to  its  actual  movement.  It 
must  be  borne  in  mind  that  in  experiment  6  we  do  not  see 
the  image  of  the  pin,  but  its  shadow ;  this  shadow  is  not  in- 
verted (as  the  image  always  is),  but  in  projecting  it  into 
space  we  invert  it  and  therefore  the  sensation  as  interpreted 
by  us  does  not  correspond  with  the  actual  condition  of 
things. 

Although  we  have  two  eyes  and  therefore  have  two  sepa- 
rate images,  yet  we  have  but  one  sensation.    The  explana- 


THE    PHYSIOLOGY    OF    VISION.  217 

tion  generally  given  for  this  phenomenon  is  the  theory  of 
corresponding  or  identical  points.'^  Suppose  L  and  R  in 
Fig.  47  are  the  left  and  the  right  retina  respectively,  and  let 
us  suppose  that  y  and  y'  are  the  yellow  spots.  These  two 
points  are  identical.  Again,  the  points  a  and  a'  are  situated 
in  the  same  direction  and  at  the  same  distance  from  the  yel- 
low spots  and  are  also  identical.  The  points  a  and  b  are 
not  identical.  It  is  held  that  if  the  images  of  a  single  object 
fall  on  identical  and  corresponding  points  we  have  one  sen- 
sation; but  if  an  object  has  its  images  on  two  non-corre- 
sponding points  of  the  retinas,  this  object  is  seen  double 
(diplopia). 

Experiment  44.  Hold  a  pencil  about  eight  inches  from 
the  face  and  another  pencil  as  far  away  as  possible.  Look 
at  the  further  pencil  and  the  nearer  pencil  is  seen  double. 
It  may  at  first  be  difficult  to  see  this ;  the  following  will 
aid  you  in  making  the  experiment.  While  looking  at  the  far 
pencil,  shut  the  right  eye  and  the  near  pencil  appears  to  be 
situated  to  the  right  of  the  far  pencil.  Now  look  at  the  far 
pencil  with  the  right  eye  and  the  near  pencil  is  situated  to 
the  left  of  the  far  pencil.  Knowing  where  the  near  pencil 
appears  to  be  situated,  look  at  the  far  one  with  both  eyes 
and  I  think  you  will  have  no  difficulty  to  see  the  near  pen- 
cil double.  The  reason  why  this  is  double  and  why  it  seems 
to  be  situated  to  the  right  when  viewed  with  the  left  eye 
can  be  gathered  from  Fig.  48. 

Let  the  far  pencil  be  situated  at  A  and  the  near  one  at  B. 
The  pencil  A  sends  a  ray  of  light  through  the  nodal  point 
(n)  of  the  left  eye,  L,  and  another  ray  through  the  nodal 
point  of  the  right  eye,  R.  These  rays  are  not  refracted,  and 
as  we  are  looking  directly  at  A,  the  images  of  A  fall  on  the 
yellow  spots,  y  and  y\  of  L  and  R.     The  yellow  spots  are 

*Identical  or  corresponding  points  of  the  two  retinas  are  points  of 
such  a  nature  that  if  they  are  simultaneously  stimulated  by  the 
images  of  one  and  the  same  object,  a  single  sensation  is  produced. 
It  is  interesting  to  note  that  this  theory  was  already  held  by 
Alhazen,  an  Arabian  mathematician  of  the  eleventh  century. 


2l8  -  THE    PHYSIOLOGY    OF    VISION. 

identical  points  and  A  is  therefore  seen  single.  B,  the  near 
pencil,  also  sends  rays  through  the  nodal  points  which  are 
focused  at  o  and  o\  These  are  non-corresponding  points, 
because  they  He  on  opposite  sides  of  the  yellow  spots  (see 
also  Fig.  47),  and  hence  we  see  this  pencil  double.  As  we 
stated  a  few  moments  ago,  we  project  the  sensation  into 
space  along  the  line  which  joins  the  stimulated  point  of  the 


Fig.  48 

retina  (b  in  L,  Fig.  48)  with  the  nodal  point  (n)  ;  hence 
the  near  pencil  is  seen  with  the  left  eye  along  the  line  bnc. 
In  projecting  our  retinal  images,  we  always  project  them 
to  the  plane  for  which  the  eye  is  focused ;  that  is,  in  Fig.  48, 
to  the  plane  mn  in  which  A  is  situated. 

In  Fig.  49  the  eyes  are  focussed  for,  the  near  pencil  B 
and  the  far  pencil  A  is  seen  double,  because  the  images  of  A 
fall  at  o  and  o'  which  are  not  identical  points.  When  these 
images  are  projected  to  the  plane  for  which  the  eyes  are 


THE    PHYSIOLOGY    OF    VISION.  219 

focussed  (mn),  it  will  be  noticed  that  the  projected  image 
for  the  left  eye  lies  to  the  left  of  B,  differing,  therefore, 
from  the  previous  experiment. 

In  this  place  I  may  draw  your  attention  to  another  inter- 
esting fact.  When  a  person  is  asked  to  hold  his  finger  in 
line  with  a  distant  object,  he  always  holds  it  in  the  line 
which  joins  the  object,  with  the  right  eye.     If  he  closes  his 


Fig.  49 

right  eye,  the  finger  is  no  longer  in  line  with  the  object. 
This  happens  if  the  observer  is  right-handed;  if  he  is  left- 
handed,  he  directs  with  his  left  eye.  A  right-handed  person 
is  also  right-eyed  and  in  daily  life  we  ignore,  no  doubt  un- 
consciously, the  images  of  the  left  eye  when  they  fall  on 
retinal  points  that  are  not  identical  with  the  points  stimu- 
lated in  the  right  eye.  This  ignoring  of  images  is  so  thor- 
oughly done,  that  some  people  find  it  impossible  to  see  the 
double  image  of  a  near  object  when  the  eye  is  fixed  upon 
A  far  object. 


220  THE    PHYSIOLOGY    OF    VISION. 

Experiment  45.  While  looking  at  an  object,  press  the 
rig-ht  eye-ball  out  of  place;  the  object  appears  double.  When 
one  eye-ball  is  moved  out  of  its  normal  position,  the  image 
of  the  object  no  longer  falls  on  the  yellow  spot  of  this  eye ; 
and  as  the  image  in  the  other  eye  does  fall  on  the  yellow 
spot,  non-corresponding  points  of  the  two  retinas  are  stimu- 
fated. 

Experiment  46.  While  looking  at  an  object  place  a  prism 
before  one  eye.  The  object  appears  double.  The  reason  for 
this  is  obvious  if  you  bear  in  mind  what  was  said  under  ex- 
periment 43. 

We  could  thus  multiply  experiments  supporting  this 
theory  of  corresponding  points,  but  time  will  not  allow.  In 
passing,  I  may  mention  that  people  who  squint  always  see 
thing's  double.  The  movements  of  their  eyes  are  not  co- 
ordinated;  when  one  eye  looks  directly  at  any  object  and 
therefore  has  the  focus  of  that  object  on  the  yellow  spot, 
the  focus  of  that  object  in  the  other  eye  does  not  fall  on 
the  yellow  spot.  Such  people  learn  to  neglect  one  of  the 
images;  but  if  a  normal  person  should  suddenly  acquire 
strabismus  by  paralysis  of  the  third  or  sixth  cranial  nerve, 
he  would  be  seriously  troubled  by  the  resulting  diplopia. 
No  doubt  you  have  noticed  that  drowsiness  is  generally  as- 
sociated with  double  vision ;  this  is  due  to  the  lack  of  co- 
ordination in  the  movements  of  the  two  eyes,  so  that  a 
slight  squint  results.  In  the  same  way  we  are  able  to  ex- 
plain the  double  vision  of  intoxication. 

The  question  naturally  presents  itself,  why  does  the  stimu- 
lation of  identical  points  cause  single  vision.  To  this  ques- 
tion there  is  no  satisfactory  answer.  It  is  assumed  bv  some 
that  it  is  due  to  the  partial  crossing  of  the  optic  nerve- 
fibers  in  the  chiasm.  From  Fig.  21  it  will  be  seen  that 
the  fibers  from  the  left  half  of  both  the  right  and  the  left 
retina  proceed  to  the  left  cerebral  hemisphere.  If  the  fiber 
which  originates  in  a  certain  spot  of  the  left  retina  ends  at 
the  same  cerebral  cell  as  the  fiber  from  the  corresponding 


THE    PHYSIOLOGY    OF    VISION.  221 

Spot  of  the  right  retina,  then  we  can  readily  understand 
how  single  vision  must  originate  when  these  two  points  are 
simultaneously  stimulated  by  the  images  of  the  same  object. 
But,  as  I  said,  this  is  almost  altogether  an  assumption. 

Are  any  objects  whose  images  do  not  fall  on  the  yellow 
spots,  and  which  we  therefore  see  by  indirect  vision,  seen 
single?    That  there  are  is  shown  in — 

Experiment  47.  Hold  a  pencil  in  the  position  indicated  in 
Fig.  50  and  look  at  the  middle  of  the  pencil  (at  a).    Both 


Fig.   50 

ends  of  the  pencil  are  then  perceived  by  indirect  vision,  the 
images  fall  not  on  the  yellow  spots  but  on  peripheral  por- 
tions of  the  retina,  yet  you  perceive  each  end  of  the  pencil 
as  a  single  object.  The  reason  for  this,  according  to  the 
theory  of  identical  points,  is  as  follows : 

In  Fig.  50  the  images  of  the  point  a,  upon  which  the 
vision  is  fixed,  fall  upon  the  yellow  spots  y  and  y',  hence 
this  point  is  seen  by  direct  vision,  is  seen  most  clearly  and 
gives  rise  to  only  one  impression.  The  end  b  of  the  pencil 
has  its  images  at  b'  and  b".  These  two  points  lie  in  the 
same  direction  and  at  approximately  the  same  distance  from 
y  and  y'  and  are  therefore  identical  points ;  hence  b  is  seen 


222  THE    PHYSIOLOGY    OF    VISION. 

single.    The  same  is  true  for  every  point  in  the  pencil,  con- 
sequently the  whole  pencil  is  seen  as  a  single  object. 

It  is  held  that  when  the  eyes  are  fixed  upon  a  point  at 
the  horizon  (primary  position  of  the  eyes)  all  points  in  the 
plane  coinciding  with  the  ground  give  rise  to  single  impres- 
sions.    This  is  known  as  the  horopter. 

Let  us  now  suppose  that  two  corresponding  or  identical 
points  are  stimulated  simultaneously  by  images  of  two  dif- 
ferent objects.  What  is  the  result?  Is  there  a  fusion  into 
one  sensation?  Generally  not.  There  is  a  retinal  rivalry, 
a  struggle  of  the  visual  fields,  as  can  readily  be  seen  from 

Experiment  48.  With  one  eye  look  at  a  yellow  card  and 
with  the  other  at  a  blue  card ;  this  can  best  be  done  by  means 
of  a  stereoscope.  Now  we  have  seen  in  experiment  23  that 
if  yellow  and  blue  are  simultaneously  thrown  into  one  eye, 
the  result  is  white;  that  is,  there  is  a  fusion  of  sensations. 
This,  however,  does  not  take  place  when  one  eye  is  stimu- 
lated by  yellow  and  the  other  by  blue.  The  observer  now 
sees  yellow  and  now  blue;  there  is  a  struggle  between  the 
two  retinas  for  supremacy. 

The  image  formed  on  the  retina  is  a  flat  image,  it  has 
length  and  breadth  but  no  thickness,  yet  in  interpreting  this 
image  we  ascribe  solidity  or  depth  to  it.  When,  for  in- 
stance, we  view  a  landscape,  we  see  this  in  its  proper  per- 
spective, and  find  no  difficulty  in  telling  near  from  remote 
objects.  As  the  image  on  the  retina  is  formed  on  a  plane 
and  has  no  depth,  it  is  evident  that  this  knowledge  of  solid- 
ity cannot  be  derived  as  such  from  the  image  in  the  manner 
it  gives  us  information  of  the  length  and  breadth  of  the  ob- 
ject; this  knowledge  is  the  result  of  many  factors.  Some 
of  the  factors  that  determine  stereometric  vision  are  mon- 
ocular, some  are  binocular.  We  shall  first  discuss  those  fac- 
tors that  are  monocular  in  origin. 

I.  Aerial  Perspective.  The  atmosphere  is  not  a  perfect- 
ly transparent  medium ;  dust  and  fog  particles  enveloping 
distant  objects  render  them  indistinct.    Whenever  an  object 


THE    PHYSIOLOGY    OF    VISION.  223 

is  thus  seen,  we  judge  it  to  be  situated  at  a  great  distance, 
whether  this  be  its  true  position  cr  not.  In  a  fog  a  near  ob- 
ject looms  very  large  because  the  indistinct  and  hazy  image 
causes  us  to  think  of  it  as  being  placed  at  a  great  distance ; 
but  as  the  image  on  the  retina  is  large,  we  over-estimate  the 
size  of  the  object.  In  a  very  clear  atmosphere,  as  in  moun- 
tainous regions,  distant  objects  are  not  as  hazy  as  at  sea 
level,  and  hence  these  objects  appear  nearer  and  smaller 
than  they  are  in  reality.  Beside  this  aerial  perspective  there 
is  a 

2.  Mathematical  Perspective.  The  retinal  images  of 
parallel  lines  are  not  parallel,  but  converging.  When  we 
stand  between  the  rails  of  a  railway,  the  rails  seem  to  con- 
verge and  meet  in  the  distance.  This  convergence  is  inter- 
preted by  us  as  associated  with  greater  distances.  The 
artist  makes  use  of  this  mathematical  perspective  to  give 
solidity  and  depth  to  his  drawing.  If  he  represents  lines 
that  are  parallel  in  nature  as  parallel  in  his  painting,  we  say 
the  painting  lacks  perspective  and  looks  fiat. 

3.  With  one  eye  we  can  tell  whether  a  given  object  is 
nearer  than  another  object  by  the  amount  of  accommodation 
necessary  to  bring  a  sharp  image  on  the  retina.  By  means 
of  the  muscle-sense  we  are  able  to  tell  whether  the  ciliary 
muscles  of  the  eye  are  more  or  less  contracted.  But  this 
is  true  only  for  objects  situated  quite  close  to  the  eye  and 
even  then  it  is  of  doubtful  value.  If  the  size  of  the  objects 
is  not  known  and  the  objects  have  a  perfectly  uniform  ap- 
pearance, that  is,  have  no  grain  or  other  surface  marks,  it 
is  almost  impossible  to  tell  which  of  the  two  objects  is  the 
nearer. 

4.  In  a  complex  retinal  picture,  like  that  of  a  landscape, 
we  judge  of  the  distance  of  various  objects  by  their  relative 
size  and  thus  obtain  a  sensation  of  solidity  even  with  one 
eye.  The  artist  makes  use  of  this  also ;  in  the  foreground 
of  the  picture  he  places  an  object  of  known  size,  such  as  a 
man  or  animal.     By  comparing  the  size  of  the  distant  ob- 


224 


THE    PHYSIOLOGY    OF    VISION. 


ject  with  that  of  the  object  in  the  foreground,  we  arrive  at 
an  idea  of  the  distance  of  the  remote  object. 

Although  we  can  to  a  certain  extent  perceive  the  sohdity 
of  objects  with  one  eye,  yet  it  is  a  well  known  fact  that 


L 


Fig.  51 


depth  value  is  more  readily  obtained  with  two  eyes.  The  rea- 
son for  this  is  sought  in  the  difference  between  the  image 
on  the  right  and  on  the  left  retina.  When  you  look  at  a 
book  lying  on  the  table,  you  see  more  of  the  right  side  of  the 


THE    PHYSIOLOGY    OF    VISION. 


225 


book  with  the  right  eye  and  more  of  the  left  side  with  the 
left  eye.    That  is,  the  two  images  are  not  exactly  the  same. 

If  one  views  a  truncated  pyramid  (A,  Fig.  51)  with  the 
right  eye,  the  image  in  this  eye  is  like  that  shown  in  R  (Fig. 
51)  ;   viewed  with  the  left  eye,  the  image  is  like  that  repre- 


Fig.  52 

sented  in  L.  If  now  you  simultaneously  cast  L  upon  the 
left  and  R  upon  the  right  retina,  you  see  the  truncated 
pyramid  stand  out  in  relief,  the  small  square  in  the  figure 
projecting  toward  the  observer.  This  can  readily  be  done 
by  means  of  a  stereoscope.  In  making  a  stereoscopic  pic- 
ture, Ihe  two  pictures  are  taken  from  two  points  of  view 
separated  by  a  distance  equal  to  the  distance  between  th^ 
right  and  left  eye.    The  left-hand  picture  of  the  stereoscopic 


Fig.  53 


view  represents  the  objects  as  they  are  seen  by  the  left  eye, 
and  right-hand  picture,  as  seen  by  the  right  eye.  By  means 
of  prismatic  lenses  the  right-hand  picture  is  thrown  upon 
the  right  retina  (the  screen  in  the  median  plane  prevents  it 
from  falling  upon  the  left  retina)  and  the  left-hand  picture 
is  thrown  upon  the  left  retina.  This  is,  therefore,  the  same 
condition  as  obtains  in  nature  when  the  tw^o  eyes  view  the 
actual  objects,  and  the  result  must  be  the  same,  that  is,  the 
objects  possess  solidity. 


226 


THE    PHYSIOLOGY    OF    VISION. 


By  a  little  practice  one  can  do  this  without  the  aid  of  a 
stereoscope. 

Experiment  49.  Look  at  L  and  R  of  Fig.  51,  but  in- 
stead of  fixing  your  gaze  upon  them,  look  through  the  page 
as  if  you  were  looking  at  a  distant  object  through  a  piece  of 
glass.  In  this  manner  four  images  are  obtained  (diplopia) 
because  the  images  of  the  figures  do  not  fall  on  identical 
points.  If  now  the  eye-balls  are  converged,  as  in  near 
vision,  the  two  central  images  overlap  and  the  pyramid 
stands  out  in  relief.  At  first  this  experiment  may  prove  a 
little  difficult,  but  the  result  is  so  striking  that  it  is  well 
worth  the  effort. 


:3 


Fig.  54 


So  delicate  is  this  process  that  by  means  of  it  we  can 
distinguish  between  a  genuine  and  a  forged  banknote.  Two 
impressions  of  the  same  plate  v/hcn  seen  through  a  stereo- 
scope produce  no  sensations  of  depth,  the  lec':ers  and  figures 
in  both  copies  coincide  exactly.  But  if  a  letter  of  the  one 
copy  is  placed  a  trifle  to  the  right  or  left  with  reference  to 
the  same  letter  in  the  ether  copy,  when  seen  with  a  stereo- 
scope, it  appears  to  be  situated  in  front  or  behind  its  mate. 

What  happens  if  the  two  pictures  are  reversed,  that  is, 
if  R  in  Fig.  51  is  thrown  into  the  left  and  L  into  the  right 
eye?  In  that  case  the  near  object  appears  more  distant  and 
the  remote  object  nearer ;  in  other  words,  a  hollow  trun- 


THE    PHYSIOLOGY    OF    VISION.  227 

cated  pyramid  with  its  base  turned  towards  the  observer  is 
seen.     This  can  be  proven  experimentally  in  B  and  C  of 

Fig.  51. 

The  question  still  remains  why  do  we  ascribe  solidity  to 
an  object  when  the  images  fall  upon  the  retinas  in  the  man- 
ner here  described.  To  this  question  no  satisfactory  answer 
can  be  given.  Some  hold  that  the  sensation  of  depth  seen 
in  a  stereoscopic  picture  is  due  to  the  muscle-sensation 
caused  by  the  contraction  of  the  internal  recti  muscles  which 


Fig.  55 


converge  the  eyes.  For  a  near  object  these  muscles  must  be 
contracted  to  a  greater  extent  than  for  a  remote  object  and 
as  we  are  conscious  of  the  extent  of  this  contraction,  we  can 
thus  judge  of  the  distance  of  an  object.  This,  however,  is 
not  a  satisfactory  explanation,  for  a  stereoscopic  picture 
gives  rise  to  the  sensation  of  solidit\^  when  it  is  illuminated 
by  an  electric  flash  of  such  a  brief  duration  that  no  muscle 
contraction  could  take  place. 

Seeing  is  a  process  of  reasoning ;  a  child  must  learn  to 
see.  A  great  many  factors  must  be  combined  before  all  the 
sensations  can  be  properly  interpreted.  This  is  well  seen  in 
children  born  blind  and  relieved  in  after-life  by  an  opera- 


225  THE    PHYSIOLOGY    OF    VISION. 

tion.  Cheselden  records  the  case  of  a  blind  boy  who  after 
the  operation  could  by  mere  sight  not  tell  which  was  the 
cat  and  which  the  dog,  although  he  knew  them  by  feeling. 
He  caught  the  cat  and  while  feeling  looked  at  her  intently 
and  said,  ''So,  Puss,  I  shall  know  you  another  time."  In 
another  case  the  person  was  unable  to  discriminate  between 
the  picture  of  an  object  and  the  real  object;  it  was  only  after 


Fig.  56 

the  object  and  its  picture  had  been  seen  and  handled  a  great 
many  times  that  he  learned  to  distinguish  them  by  sight. 
Yet  the  retinal  images  of  the  object  and  its  picture  were 
exactly  the  same  as  in  a  normal  person ;  he  lacked  not  the 
sensation  of  sight  but  the  ability  to  interpret  the  sensation. 
There  w^as  not  that  knowledge  which  we  derive  from  simul- 
taneously seeing  and  touching  the  objects.  Somebody  has 
well  defined  seeing  as  feeling  at  a  distance. 

In  our  remarks  about  solidity  we  have  already  referred 
to  our  ability  to  judge  of  distance.  In  objects  situated  at  a 
great  distance  from  the  eye,  aerial  perspective  plays  a  great 
part.     For  this  reason  distant  parts  of  the  landscape  seem 


Fig.  57 

nearer  and  smaller  in  wet  than  in  dry  weather ;  the  dust  of 
the  atmosphere  increases  the  aerial  perspective  and  because 
of  this  haziness  the  objects  are  judged  to  be  situated  at  a 
greater  distance  than  they  in  reality  are  and  therefore  the 
size  of  the  objects  is  also  over-estimated. 

The  size  of  the  retinal  image  is  used  in  our  judgment  of 
distance;  if  the  image  is  that  of  a  known  object,  we  can 


THE    PHYSIOLOGY     OF    VISION.  2  2^ 

quite  accurately  estimate  the  distance  between  us  and  the 
object,  but  if  the  size  of  the  object  is  not  known  it  may 
lead  to  gross  error.  The  reason  for  this  can  be  gathered 
from  Fig.  52.  The  objects  a,  b,  and  c  all  have  the  same 
sized  image  (xy)  on  the  retina.  If  I  know  the  size  of  a, 
but  not  that  of  b  or  c,  I  might  conclude  that  all  three  ob- 
jects are  at  the  same  distances  from  the  eye.  Suppose  c 
is  a  cow  and  suppose  a  is  a  cat ;  the  images  of  both  animals 
are  of  the  same  size,  but  as  I  am  familiar  with  the  size  of 
these  animals,  I  conclude  from  the  size  of  the  images  that 
the  cow  is  much  further  away  than  the  cat.  How  mislead- 
ing this  can  be  is  seen  from  the  story  of  the  farmer  who 

^-^  > < 

Fig.  58 

called  for  his  gun  to  shoot  a  chicken-hawk  when  he  saw  a 
fly  crawling  on  the  window.  He  was  looking  at  the  sky  and 
mentally  placed  the  fly  at  this  great  distance  and  therefore 
greatly  over-estimated  its  size.  As  he  was  not  focussing 
for  the  fly,  the  image  on  the  retina  was  blurred ;  all  th's 
aided  him  in  his  delusion. 

We  have  already  alluded  to  the  fact  that  it  is  extremely 
difficult  to  judge  of  distances  with  one  eye. 

Experiment  50.  Close  one  eye  and  attempt  to  thread  a 
needle  held  at  a  distance  of  about  eighteen  inches.  It  will 
be  fcund  no  easy  task,  especially  if  the  distance  of  the  needle 
is  varied.  An  interesting  variation  of  this  experiment  is  as 
follows :  Make  a  small  mark  on  a  piece  of  paper  lying  on 
the  table.  Close  one  eye  and  try  to  strike  the  mark  with  the 
end  of  a  pencil.  What  is  extremely  easy  with  two  eyes  be- 
comes difficult  if  one  eye  is  closed. 

That  our  judgment  of  size  is  governed  by  our  idea  of  dis- 
tance is  evident  from  the  projection  of  the  negative  after 
image. 


:3o 


THE    PHYSIOLOGY    OF    VISION. 


Experiment  51.  Look  at  a  distant  window  for  some  time 
so  as  to  obtain  a  negative  after  image.  When  you  think 
this  has  been  obtained,  look  at  a  piece  of  paper  held  within 
a  foot  from  the  face ;  the  image  of  the  window  looks  small. 
Now  look  at  the  ceiling  or  distant  wall  and  a  large  image 
of  the  window  is  seen.    Yet  the  image  of  the  window  in  the 


i^Mg.  59 


eye  is  of  the  same  size  in  both  cases ;  our  conception  of  size 
depends  largely  on  the  distance  to  which  we  project  the  sen- 
sation. 

Our  judgment  of  distance  is  greatly  modified  by  many 
other  factors.  To  most  people  the  distance  from  b  to  c  in 
Fig.  53  appears  greater  than  that  from  a  to  b,  yet  they  are 
equal.  We  can  judge  more  accurately  of  the  distance  be- 
tween two  points  when  several  objects  intervene;  a  lands- 


THE    PHYSIOLOGY    OF    VISION. 


231 


man  is  a  poor  judge  of  distances  at  sea.  The  moon  at  the 
horizon  appears  larger  to  us  than  when  at  the  zenith;  the 
many  intervening  objects  between  us  and  the  moon  when  it 
is  at  the  horizon  give  us  an  idea  of  greater  distance  and 
therefore  of  greater  size.  For  this  reason  also  we  think  of 
the  sky  as  a  flattened  dome.  When  we  were  boys  we  amused 
ourselves  by  viewing  the  landscape  or  street  with  our  head 
between  our  knees.     Because  of  the  nearness  of  the  objects 


M 


\ 


\ 


Fig.  60 

in  the  foreground,  which  are  neglected  when  we  are  in  the 
erect  position,  we  judge  of  the  further  objects  as  situated 
at  an  immense  distance. 

In  judging  of  the  distance  between  b  and  c  in  Fig.  53,  we 
mentally  add  the  smaller  distances  between  the  intervening 
points  and  the  result  is  greater  than  when  the  mind  must 
grasp  the  whole  distance  between  a  and  b  at  one  jump.  In 
Fig.  54  the  height  of  A  appears  greater  than  its  length, 
while  the  reverse  is  true  for  B.  That  A  and  B  are  perfectly 
square  and  of  the  same  size  is  hard  to  believe.  It  may  also 
be  stated  that  the  space  between  A  and  B  is  of  the  same  size 
as  A  or  B,  although  it  appears  considerably  smaller.  The 
reason  for  this  optical  illusion  is  complex.  First,  we  have 
this  mental  summation ;  we  add  the  several  vertical  distances 
in  A  and  hence  it  appears  higher  than  it  is  long  and  also 
higher  than   B.     Again,   we  always  over-estimate   vertical 


232  THE    PHYSIOLOGY    OF    VISION. 

length ;  the  vertical  line  in  Fig.  55  appears  longer  than  the 
horizontal  line.  For  this  reason  the  difference  between  the 
height  and  the  length  of  A  is  greater  than  the  difference  be- 
tween the  height  and  length  of  B.  There  is  a  third  reason. 
In  Fig.  56  the  two  lines,  a  and  b,  do  not  appear  to  be  on  a 
level ;  a  appears  to  be  situated  higher  up  than  b.  The  pres- 
ence of  the  line  below  a  raises  a  and  the  line  above  b  de- 
presses b.  This  is  also  true  for  the  horizontal  lines  in  A, 
Fig"-  54;  they  seem  to  force  each  other  apart  and  thereby 
increase  the  height  of  A. 

Another  factor  that  greatly  influences  our  judgment  of 
distances  and  size  is  the  presence  of  angles.     In  Fig.  57  the 


Fig.  61 

line  a  is  judged  to  be  longer  than  the  line  b.  Similarly  it  is 
difficult  to  believe  that  the  line  B  in  Fig.  58  is  no  longer  than 
the  line  A.  Because  of  the  presence  of  the  short  diagonal 
lines  in  Fig.  59,  the  heavy  black  lines  seem  to  diverge  and 
converge  alternately.  The  degree  of  convergence  and  diver- 
gence depends  to  a  large  extent  upon  the  position  of  the 
parallel  lines.  If  the  book  is  held  in  such  a  position  that 
the  long  lines  in  Fig.  59  are  vertical,  the  lines  appear  more 
nearly  parallel;  this  is  still  more  evident  if, the  lines  are  held 
horizontally. 

For  this  reason  also  the  three  short  oblique  lines  in  Fig. 
60  do  not  seem  to  lie  in  a  straight  line ;  the  middle  piece 
seems  to  be  placed  lower  than  the  upper  and  higher  than 
the  lower  piece.  This  illusion  also  disappears  to  a  large  ex- 
tent if  the  line  is  held  horizontallv. 


THE    PHYSIOLOGY    OF    VISION. 


233 


In  conclusion  I  may  call  your  attention  to  another  optical 
deception.  In  Fig.  61  we  have  Necker's  parallelopiped. 
When  the  corner  a  is  viewed,  the  parallelopiped  generally 
seems  to  lean  towards  the  observer  and  the  end  F  faces  him. 
After  looking  at  the  figure  for  a  few  moments,  especially 
when  the  point  b  is  fixed  upon,  the  figure  seems  to  change 
and  the  parallelopiped  leans  away  from  the  observer.  Still 
more  beautifully  is  this  illusion  brought  out  in  Fig.  62.  If 
this  figure  is  looked  at  it  may  be  seen  either  as  two  cubes 


Fig.  62 


resting  upon  another  cube  with  the  under  surfaces  dark,  or 
as  one  cube  resting  on  two  cubes  with  the  upper  surface 
dark.  The  mathematical  perspective  is  such  that  both  inter- 
pretations are  possible. 

From  all  this  it  is  very  evident  that  seeing,  by  which  we 
mean  the  interpretation  of  the  various  sensations,  is  an 
extremely  complicated  process  and  one  that  may  frequently 
lead  us  to  false  conclusions,  so  that  it  is  often  a  question 
whether  cr  not  we  can  trust  our  own  eves. 


INDEX  OF   ILLUSTRATIONS. 


A  moeba 153 

Arteria  Centralis  Ketina 

22,65,90,119 

Arteries,  Ciliary 22,  €2,  89 

of  Lids 48 

"  and  V^eins  of  Orbit.    59 


Benliam's  Disk 183 

Blind  Spot Irt9 

Blood  Vessels  of  Iris 62,  64,  95 

Bones  of  the  Orbit 42 

Bowman's  Membrane,  33,80,81,  83 

C 

Canal  of  Schlemm 

33,  80,  83,  93, 104,  103, 106 

Caniculi,  Corneal 81,83 

Capillaries  of  Skin 87 

"  S  u  r  r  o  un  ding 

Cornea 80,  t3,  87 

Center,  Reflex 154 

Ciliary  Arteries 33,62,89 

Bodies 33,9<{,93, 

91,  102, 104,  105,  106, 107,  146 
"         Bodies,     Develop- 
ment of.... 24,  25,26,27,29 

Nerves 68,70 

"         Muscles 146 

"         Processes 

...33,92,93,104,105,106,107 

Choroid 33,89,90,91 

Choroidal  Fissure 33,  89,  90 

King 33,89,90 

Circulus  Major 

...33,  83,  93,95,100,104,106 

Minor 93,95,100 

ofZinn 33,89,90 

Color  Sensations,  Diagram  of  198 

"      Triangle 195 

Cones  of  Ketina 1G7 

Conjugate  Foci 132,134 

Conjunctiva,  Development  of 

...  24,  25,  26,  27,  29,  53,  54,  56,  57,  80 
Cornea.  Development  of  12, 13, 
14, 18,  22,  23,  24,  25,  29,  33,  80,  81,  83 

Corneal  Caniculi 81,  S3 

"         Corpuscles 81,83 

Endothelium 81,83 

Corpuscles,  corneal 81,  83 

Corrugator  Supracilia..  ..45,53,  54 


Cross  Section  of  Eyeball 33 

*'           "          *'         "          and 
Orbital  Structures 56,57 

Crystalline  Lens,  see  Lens. .. 

D 

Decimet's  Membrane 81,  83 

Development  of  Ciliary 

Bodies 24,  25,  26,  27,  29 

Development  of  Conjunctiva 
24,  25,  26,  27,  29,  53,  54,57, 80 

Development  of  Cornea 

12, 13, 14,  IS,  22.  23,  24,  '^,  29,  80,  81 

Development  of  Iris 27,  29 

"  of  Lens 

11.12,13,14,15,16,17 

Development  of  Lids  or  Pal- 
pebrarum   

12,  13,  14,18,23,23,24,25 

Development  of  Plica  Semi- 
lunaris  22.23,24,26 

Development  of    Recti  Mus- 
cle.^  22,  24.  24,  25,  C6 

Deve'opment  of   the    Retinn 
9  10. 11.  12.  1.3,  14, 18.  22,  23.  24.  ^5. 
26,  27, 33,  65.  89,  90, 106,  107, 113,  116 

Development  of  Suspensory 
Ligament,  or     Zonule    o"f 

Zinn 13,14.18.22,23, 

26.  33,  93,  99,  101.  104,  105,  106,  107 

Development  of  Vitreous 

13,  14,18,22,26 

E 

Endothelium,  Corneal 81,  83 

Entoptical  Vision 159,165 

External  Structures  of  Eye. .    41 

Eyeball,  Cross  Section  of 33 

Eye.    Diagrammatic  Section 


of. 


.145 


Focus,  Position  of 133,134 

Folding  of  Optic  Stalk    and 

Vesicles 16,  17 

Fossae  Patellaris 108 


Glands,  Ilenle's 76,77 

"         Krause's 79 

'•         Lachrymal 53,79 

Meibomian  ..  .53,  72,  73,  74 
"         Moll's 72,73.74 

Waldeyer's 72,78 

•*        Ziesse's 72,73,74 


^34 


PHYSIOLOGY    OF    VISION. 


235 


Hair  Follicles 73 

Henle's  Glands 76,77 

HyaloidArtery 22 

"  Canal    (Canal   o  f 

Stilling) 108 

"  Membrane 108 


Identical    Points,     Diagram 
Illustrating 216,221 

Image,  by  Convex  Lens 131 

"       inEye 136,137 

Inter  Vaginal  space. 89,  90,  119, 120 

Iris 27,  29, 

33,  95, 98,  99,  100,  101,  102,  104,  107 


Membrane,  Decimet's. . .  .80,  81,  83 
Pupillary. ...19,  24,29 

Moll's  Gland- 72,  73,  74 

Muscae  Volitantes 165 

Muscles,  development  of  Rec- 
ti  22.  23,24,25,26 

Muscle  of  Riolanis 72,  73.  74 

N 

Necker's  Parallelopiped 232 

Negative  after  irimge,  diag- 
ram illustrating 208 

Nerves  Ciliary 68,  70 

"       ofLids 5a 

Optic .  .33,  89,  90, 119, 120, 121 
"  '*       development  of 

13,14,16,22,23,24,26 

of  the  Orbit 67,6* 


Kraus'  Glands 


79 


Lachrymal  Apparatus 52 

Gland 47,53,79 

Lacunae,  Corneal 81,  83 

Lamina  Cribrosa 33,  89,  90 

Propria 80,81,83 

Lens,   changes  in  during  ac- 
commodation   147 

"      structure  of ir3,164 

"      deve'opment  of 

11, 12, 13,  i4, 15, 16, 17 

••       Pit 10,11 

"      Vesicle 12,13 

Lenses,  refraction  of  light 

by 131,  134 

Lids,  or  Palpebrarum,  deve- 
lopment (if ..  .12, 13, 14, 18.22. 

23,24,  25 

"     Arteries  of 48 

"     Cross  section  of 72,80 

"     Nerves  of 50 

"     Veins  of 49 

Ligament  of  Lockwood 47 

Pectinate 33,83,93 

Light,  Passing  thro'  lens 131 

Reflection  of 130 

Refraction  of 131 

Limbus 33,80,83 

M 

Macula  Lutea 33,  65,  89 

Meibomian  Glands ....  52,  72,  73,  74 
Membrane,Bowman's,33,80,81,  83 


O 


Optic  Nerve.. 33,  89,  90, 119, 120, 121 
"  *'       development    of 

. .  13,  14, 16,  22,  23,  24,  26 
Sheath.. 22,  23,  24, 

26,89,90,  119,120 

"       Stalks,  Primary » 

"  "  and    Vesicles 

Folding  of.  ..16, 17 

'*       Vesicles,  Primary 9 

"             "           Secondary...    10 
Ora  Seratta, 33, 92. 93, 104, 105, 10(5, 107 
Orbicularis  Palpebrarum,  46, 
56,  57 


Palpebrarum,  developm  e  n  t 

of 12, 13, 14, 18, 1.'2,  23,  24,  25 

Papillae,  Post  Tarsal 76,  77,  78 

Pectinate  Ligament 33,  83,  03 

Perimetric  Chart  of  Retina..  207 

Physiological  Cup 33,  89,  90 

Pia  Mater,  of  Optic  Nerve. 89. 

90,  119,120 

Plica  .Semi-Lunaris,  develop- 
ment of 22,23,24,26 

Post  Tarsal  Papillae 76,77,78 

Primary  Optic  Stalks 8 

"  "      Vesicle 9 

Principal  Focus,  Position  of.  133 

in  Eye 13J 

Prism,  Effect  of,  on  Projec- 
tions   216 

Pupil 33,  101,  107 

Pupillary  Membrane 19,24,29 

Purkinje-Sanson  Images.  .141, 143 


236 


ANATOMY    OF    THE    EY! 


Recti   Muscles,  Anterior  At- 

tactitiient  of.    54 
"  "  Development 

of..  22,  1:3,24,25,26 
"  "  from  Above..    58 

Reflection  of  Li^^lit 130 

ReflexArc 154 

Refraction  of  Li^jht 131-134 

Reiina.  Development  of  0.  1(», 
11,  12,  13. 14,  18.  22.  23,  24  2.j.  !:(i, 
27,  33,  65,89.  90,  106,  107,  113,  116 

Retina.  Structure  of 167 

Riolanis.  Mtiscle  of.. 72,  73,74 

Rods  of  Retina i. 167 

8 

Scleral  Ring 33,  89,  90 

Secondary  Optic  Vesicles....    10 
Sensation,  Visual,  Curve  of..  175 

Spherical  Aberration 151 

Stalks,  Primary  Optic 8 

Stereometric    Vision,    d  i  a  - 

grams,  illustration  of 224 

Stratified       Epithelium      o  f 

Cornea 33,80,81,83 

Suspensory  Ligament,  Zon- 
zule  of  Zinn,  Develop- 
ment of 13, 14. 18.  22,  23, 

26,  33,  93,  99,  101, 104, 105.  106  107 


T 

TendoOculi 45,47 

V 

Veins  and  Arteries  of  Orbit..    59 

••       of  Eyeball 64 

"    Lids 49 

Vena  Centralis  Retina 

33,  65  90,119 

Vesicles,  Lens 12, 13 

Optic,  Folding  of..  16, 17 
"  "         Secondary  ..     10 

"  Primary  Optic 9 

Vitreous  Body 108 

"  Development  of .    ... 

13,14,18,22,26 

W 

Waldeyer's  Glands 72,78 

Wliirling  Machine 187-188 

Z 

Ziesse's  Glands 72,73,74 

Zinn.Circulus  of 33,89,90 

ZoUner's  Lines 230 

Zonule  of  Zinn  (.Suspensory 
Ligament  13,14.18,22,23, 
26,  33,  93,  99,  101,  104,  105, 106, 107 


INDEX. 


A 

Achromotopsia 204 

Accommoda'-.ion 139-149 

"  unequal 150 

"  action  of  drugr^  on.  158 

"  pupil  constriction 

during 151,162 

Actinic  rays 191 

After-image,  positive 186,190 

negative..208,  210,  229 

Alcohol 158 

Angular  Artery. 49 

Vein 50 

Anterior  Cerebral  Vesicles. ..      9 

Chamber 34 

"  Ciliary  Arteries..  .35,  60 

Veins 36 

"  Corpora     Qnadriye- 

mina 122 


Anterior  Temporal  Vein 50 

Arachnoidal  Sheath 122 

Arteria  Centralis  Retina.  16, 

38,60,64 

Artery  Hyaloid 22 

Astigmatism 149 

Atropin 158 

Axis  Cylinder  Processes.. .. .  118 

B 

Benham  spectrum  top 182 

Blindness,  psychical 173 

Blind  spot 168,  169 

Blood-corpuscles  seen  in  eye.  166 
"       vessels  seen  entopical- 

ly 165 

Bowman's  Membrane 17,82 

Bruck's  Membrane 93 


AND    PHYSIOLOGY    OF    VISION. 


237 


Calcarian  Fissure 123 

Canal  of  Petit 34.40,103 

"     ofSciilemm 32,36.88 

"     ofStilling 22 

Caniculi 84 

Canthi 43 

Capillary  Loops 88 

Capsule  of  Tenon 86 

Caruncle , 43 

Cavernus  Sinus 61 

Charpentier's  Bands.  183 

Check  Ligaments 56.58 

Chemical  Rays J91 

Cliorio  Capillaris 92 

Choroid 25,27,28,35,90 

Choroidal  Fissure 15, 16,35 

Chromatic  Threshold 182 

Cilia,  or  Eyelashes 28.44 

Ciliary  Body 30,32,94 

"       Ganglion 37,69 

"       Muscle 

32,35,106,113,146,148 

"       Nerves 37 

Plexus 37,  70,  lOa 

Processes 32,94 

Circulus  Major 33,  63, 1)7 

Minor 36,63,97 

of  Zinn 37,  C3 

Colors,  Threshold  of 182 

Properties  of 1P3 

Combinations  of.   194,  203 

"        Intensityof 194 

"        Complementary 195 

"        Primary .-.. 197 

Color  Blindness 204 

"      Constants 193 

"      Triangle 195 

Colored  Shadows 212 

Complementary  Colors 195 

ConesandRods 114 

of  Retina 103,171 

Conjugate  Foci 132 

Conjunctiva 28,  79 

Conjunctival  Sac 27,  28,  30 

Consensual  Pupil  Reflex.   ...  156 

Contrast,  Successive, 208,210 

"  Simultaneous 211 

Cornea 13,15,17,30,81 

Corneal  Corpuscles 84 

"         Epithelium 82 

Corresponding  Points  of  Re- 
tina    217 


Corrugator     Supracilia 

Muscles 46 

Crystalline   Lens,     Index    of 

Refraction 135 

Crystalline  Lens,  Curvature.  136 
"    .  "        Changes 

During   Accommodation 

144.150 

Cry.^taliine  Lens,  Hlasticitv 

of 144,  148 

Crystalline  Lens,    Structure 

of....: 163,  164 

Cuniate  Lobe 123 

I) 

Daltonism. 206 

Deceptions,  Optical ...231-233 

Decimet's  Membrane 85 

Decussation  of  Optic  Nerve.. 

156,1  8,220 

Defects  of  Vision 148, 149, 150 

Depth  Value  of  Visual  Sensa- 
tions    222,  227 

Dicliromatipm 205 

Differentiation,     Physiologi- 
cal      153 

Dilator  Pupillae  Mupcle 31 

Diopter 13,') 

Diplopia 217 

Distance,  Our  Judgment  of..  228 
Double  Vision 217 

E 

Etbemoidal  Fornmen 41 

Endothelium 85 

Emmetropia 139 

Entoptical  Vision 1.58-166 

Ethemoidal  Arteries 61 

Ether  Vibrations* 191,192 

External  Geniculate  Body. ..  122 

Eyeball,  Dimensions  of 30 

Eye,  Radii  of  Curvatures  of..  1.36 

"      Refractive  Media  of 135 

"     Refraction  of  Light  in... 

136,137 

F 

Fibres  of  Mueller 21.  US 

Filtration  Angle 32,  85 

Flicker  Phenomenon 183 

Flies,  see  Muscae  Volitantes 
Focus,  to  Find  Position  of  .132-134 

Fornix  Conjunctiva 28,  58,  80 

F.).esae  Patellaris 31 

Fovea  Centralis.. 38,  66, 169, 177,  207 


238 


ANATOMY    OF    THE    EYE 


Frontal  Artery 49  61 

Vein 50 

Fusion   of   Retinal    Impres- 
sions  187-189 

G 

Ganglionic  Layer 118 


H 

Helmholtz  Theory  of  Accom- 
modation    144 

Helmholtz  Theory   of   Color 
Sensations 198 

Hering  Theory  of  Color  Sen- 
sations   199,  209,  213 

Heme's  Glands 77 

Holmgren  Method  for  Deter- 
mining Color  Blindness. .  203 

Horner's  Muscle 48 

Hyaloid  Artery 22 

Canal 22,  109 

Membrane 32,  40,  109 

Hypermetropia 148,  149 


217 
233 


160 
134 


135 


Identical  Points  of  Retina. .. 

Illusions,  Optical 231, 

Image,  Position  and  Size  of 
Retinal  Image,  137, 138, 159, 

Index  of  Refraction 

"  "  of  >[edia 

of  Eye 

Inferior    Internal    Palpebral 

Artery 50 

"        Nasal  Artery 38 

"         Oblique  Muscle 55 

"         Palpebral  Vein 50 

*'        Temporal  Artery....    38 

Infra  Orbital  Artery 49 

"  "         Foramen 42 

"  "         Nerve 51,  69 

"  "         Vein 50 

"        Trochlear  Nerve 51.68 

Inner  Limiting  Membrane. ..  118 

"       Molecular  Layer 117 

"       Nuclear  Layer 117 

Internal  Geniculate  Body....  122 
Inter  Vaginal  Space..  .57,  90,  121 

Intrinsic  Light  of  Retina 166 

Invagination  of  Primary  Op- 
tic X'esicle 11 

Iris 28,30,35.44.97 

"    Function  of 151-168 

"    Muscles  of 157 

Irradiation 184 

Irritability MS,  176 

Threshold  of 177 


K 

Kraus'Glands 78 

L 

Lachrymal  Artery 49,60 

'■         Canal  or  Tear  Duct 

14,4L51 

Canaliculi 51 

Gland 48.53,78 

Nerve 51,  67 

Papillae 44,52 

Lachrymal  Puncta 44,  52 

Sac 51.52 

Lacuna,  Corneal 30,  84 

Scleral 87 

Lakus 43 

Lamina  Cribrosa 25,34,39,89 

Fusca 86 

Propria 17,30,82 

Lens 13.15,17,29,33,99 

"      Capsule 33,  102 

"      Stars 20 

Lenses,  Refraction  of 131 

Lenticular  Ganglion 69 

Levator  Palpebra  Superioris. 

30,  47,59 

Lids,  Formation  of 14, 17 

Ligament  of  Lockwood 47 

ofZinn 47,60 

Light,  Action  on  Iris 152 

"  "         "     Rhodopsin 

171,172 

"        Number  of  Vibrations 

191,192 

Reflection  of 129-130 

Refraction  of    130-131 

Velocity  of 130 

Limbus 34 

M 

Macula  Lutea 38,  66 

Mariotte's  Experiment 169 

Meibomian  Glands 54,  75 

Membrana  Nictatans 21 

Mesoblast 13 

Moll'sGlands 75 

Morphine 158 

Mueller  Fibres  of 21 

Muscae  Volitantes 164 

Mu.scles  of  Accommodation 

145,146 

of  Riolanus 72,75 

Mydriation J58 

Myelin  Sheaths 39 

Myopia 148,  149.164 

Myotics 158 


AND    PHYSIOLOGY    OF    VISION. 


239 


N 

Nasal  Nerve 67 

Near  Point  of  Visit)n 147-149 

Necker's  Parallelopiped 232 

Negative  After  Image 208,  210 

Nerve  Fiber  Layer 118 

"      Impulse 173 

Neural  Tube 9 

Nodal  Point  of  Lens 134 

"      of  Eye 137,215 

Nutrient  Lymph 32 

O 

Occipital  Lobes 173 

Ocular  Conjunctiva 28,  80 

Oculo-Motor  Nerve 150, 158 

Old-Sightedness 149 

Ophthalmic  Artery 60 

Vein 61 

Optic  Commisure 122 

Disk 168 

"        Foramen 41 

Nerve 26,118 

"  "      Decussation  of.. 

156,  108,220 

"  "      Origin  of 168 

**      Sheath 

28,34,57,121 

"       Radiations 123 

Thalmus 122 

Tracts 122 

Optical  Illusions 231-233 

Optogram 172 

Ora  Serratta 32 

Orbicular  Ligaments 46 

Orbit  of  the  Bye 41 

Orbital  Fat 57 

Nerve 69 

Orbicularis  Palpebrarum 

Muscle 28,46 

Outer  Limiting  Membrane. ..  110 
"     Molecular  Layer 117 

P 

Pa  1  pebrae 43.  71 

Palpebral  Conjunctiva 28,  80 

Fisdure 43 

Pectinate  Ligament 32,  85 

Perimeter 207 

Perspective 222,  226 

Phos phene 164,  215 

Physiological  Cup 39 

Phvsostigmin 158 

Pia  Mater 90 


Piai  Sheath 122 

Pigment  Layer  of  the  Retina.  113 

Plexus  of  Mises 51 

Plica  Semilunaris 21,27,  28,43 

Porus  Opticus 39,  89 

Positive  After-image 186-190 

Posterior  Chamber 34 

"  Ciliary  Arteries 

35,59,60,62 

"  Ciliary  Nerves 69 

Post  Tarsal  Papillae 77 

Presbyopia 149 

Primary  Optic  Stalk 9 

Principal  Focus 132 

Prism,  Displacement  of  Ob- 
ject Seen  Through...  216 
"        Double  Vision  with..  220 

Projection  of  Sensations 214 

Pulse,  as  seen  in  pupil 158 

Purkinje's  Phenomenon 194 

Purkinje-Sanson  Images 141 

Pnpil 31.44 

Effect  of  Light  on 152 

"        Effect  of    Near  Vision 

on 151 

Constriction  of  ..152, 155, 158 

"        Reflex 156 

Pupillary  Membrane 24,30 

B 
Recti,  (Extrinsic)  Muscles.. 25, 28 

Reflection  of  Light 129, 130 

"  Image  of,  in  Eye..  141 

Reflex  Ac<  ion 152 

"         Center I'l 

Refraction  of  Light 130, 131 

Refractive  Media 40 

"  P«jwer  of  Lenses..  13t 

Retina HO 

**       Blood  Vessels  of 165 

"         Corresponding 

Points  of 217 

"         Formationof.11,15,  29,  33 
"         Intrinsic  Light  of....  166 

"         Structure  of 167 

Retinal  Rivalry 222 

Rhodopsin 171, 172, 178 

Right-Handedness 219 

Rods  and  Cones 114 

"      of  Retina 168,171,177 

S 

Saturation  of  Colors 193 

Scleral  Endothelium 86 

Schneiderian  Gland 43 


240 


ANATOMY    OF    TTIE    EYE 


Sclero-Corneal  SuIpus 31 

Sclerotic 21,  27,  28,  34,  8t> 

Secondary  Optic  Vesicle 11 

Sensations,  Projection  of 214 

Sense  Organs,  Object  of 1?9 

Sitnultaneous  Contrast 211 

Single     Vision    with     Two 

Eyes 216,  221 

Size,  Judgment  of 22S 

Source  of  Material  for  Illus- 

1  rations 5 

Space  Sensations 214.  233 

Spaces  of  Fontana 32,  85 

Space  of  Tenon 26,  56,  86 

Specific  Energy  of  Nerve 173 

Spectrum  of  Solar  l^ight 192 

Sphenoidal  Fissure 41 

Sphincter  Pupiilae  Muscle. ..    31 

Spherical  Aberration 151 

Squinting 220 

Stars,  Why  not  Visible  Dur- 
ing Day l<il 

Stereometric  Vision 222 

Stereoscope 225,227 

Stimulus 153 

"       Adequate 191 

"       Duration  of  Visual 

Stimulation 174 

"       Liminal     Intensity 

of 176 

Strabismus ..  220 

Successive  Contrast 208-210 

Superior  External  Palpebral 

Artery 50 

"  Internal  Palpebral 

Artery 50 

"  Maxillary  Nerve..    42 

'*  Oblique  Muscle —    44 

Nasal  Artery 38 

"  Oblique  Muscles..    59 

"  Palpebral  Vein —    50 

"  Rectus  Muscle —    59 

Temporal  Artery.     38 

Supra  Choroidal  Space... 35,  40,  86 

Cilia 45 

Orbital  Artery 49,  60 

'•       Foramen 43 

"       Nerve 51,  68 

Trochlear  Nerve 51,  68 

Vaginal  Space 57 

Suspensory  Ligament 

21,32,40.101,144,146 

Sympathetic  Nerve  of  Iris. . .  158 


Tarsal  Cartilages 47,  54,  71 

Tendo  Oculi 51 

Tenon's  Capsule 56 

Tenon,  Space  of 27,86 

Tensor  Tarsi  Muscle 49 

Threshold  of  Light  Stimulus  176 
*'            "  Color    Stimula- 
tion   182 

Trabeculae 35,86,  122 

Transitional  Zone 100,102 

Troc  h  1  ea 47 

Tscherning's  The'>ry  of    Ac- 
commodation      147 


U 


Ultimate  Elements  of  Sight 

168,  171 

Uvea 110 

Uveal  Coat 35 


Vena  Centralis  Retina 

"     Vorticosa 36.61 

Vision  Requisites  for 

"'       Sensations  produced 

by 

Visual  Angle 

Purple 171, 

"       Sensations 

"  "         Duration  of 

"  "         Increase  in 

Intensity  of 

"  "         Project  ion 

of 

Vitreous  Body  Formation  of 
13,15,  21,30,39, 


Humor. 


39 
,63 
139 

173 
170 
172 
173 
175 

175 

214 

109 
164 


\V 


Waldeyer's  Glands 78 

Weber's  Law 178,131 

Winking  Membrane 21 


Yellow  Spot 38,  169 

Z 

Zeisse's  Glands 72,  75 

Zonule  of  Zinn 21,  32 


